UNIVERSITY   OF   CALIFORNIA 

COLLEGE    OF    AGRICULTURE 

AGRICULTURAL    EXPERIMENT    STATION 

BERKELEY,  CALIFORNIA 


ORCHARD  HEATING  IN  CALIFORNIA 

WARREN  R.  SCHOONOVER  AND  ROBERT  W.  HODGSON 

IN  CO-OPERATION  WITH  FLOYD  D.  YOUNG 
OF  THE  UNITED  STATES  WEATHER  BUREAU 


BULLETIN  398 

December,  1925 


UNIVERSITY  OF  CALIFORNIA  PRINTING  OFFICE 

BERKELEY,  CALIFORNIA 

1925 


Digitized  by  the  Internet  Archive 

in  2012  with  funding  from 

University  of  California,  Davis  Libraries 


http://www.archive.org/details/orchardheatingin398scho 


ORCHARD  HEATING  IN  CALIFORNIA 

By  WAKKEN  E.  SCHOONOVEKi  and  KOBERT  W.  HODGSON,2 
In  cooperation  with  FLOYD  D.  YOUNG3 


FOREWORD 


Winter  and  spring  temperatures  sufficiently  low  to  injure  orchard 
trees  have  caused  losses  to  California  fruit  growers  amounting  to 
millions  of  dollars.  The  importance  of  orchard  heating  as  a  necessary 
operation  in  successful  fruit  culture  in  many  parts  of  the  state  is 
therefore  now  more  fully  realized,  particularly  for  certain  sub-tropical 
fruits.  The  numerous  demonstrations  of  the  utility  and  economic 
practicability  of  orchard  heating  which  have  been  made  are  respon- 
sible for  a  marked  increase  in  the  use  of  orchard  heating  equipment — 
more  than  900,000  heaters  having  been  purchased  by  California  Citrus 
fruit  growers  alone  during  the  years  1922-25. 

As  a  result  of  this  increased  interest  in  orchard  heating  there  has 
been  an  unprecedented  demand  on  the  College  of  Agriculture  for 
information  concerning  methods  and  costs  of  providing  protection 
against  damaging  temperatures. 

Although  the  College  of  Agriculture  has  given  some  attention  to 
field  studies  of  this  practice,  no  comprehensive  investigations  of 
orchard  heating  have  been  conducted  by  the  Agricultural  Experiment 
Station.  However,  Floyd  D.  Young  of  the  United  States  Weather 
Bureau  has  been  studying  this  subject  for  some  years,  and  fortunately 
for  California  fruit  growers,  much  of  the  work  has  been  done  in  this 
state.  A  highly  efficient  fruit  frost-forecasting  service  has  been 
developed  and,  in  financial  cooperation  with  certain  groups  of  fruit 
growers,  much  valuable  information  bearing  on  orchard  heating 
methods  and  equipment  has  been  accumulated. 

In  order  to  make  this  information  available  this  year  at  the 
season  when  the  need  is  particularly  urgent,  a  cooperative  arrange- 
ment was  undertaken  last  fall  providing  for  a  survey  by  the  College 
of  orchard  heating  practices,  costs,  and  results,  and  the  preparation 
under  joint  authorship  of  a  manual  of  instructions  in  orchard  heating 
for  the  use  of  California  fruit  growers.  The  valuable  assistance  of 
Mr.  Young  in  the  preparation  of  this  publication  is  hereby 
acknowledged.  E   D   ^^ 

Director  Agricultural  Experiment  Station, 
College  of  Agriculture,  University  of  California. 


1  Extension  Specialist  in  Citriculture. 

2  Associate  Professor  of   Sub-tropical   Horticulture   and  Associate   Citricul- 
turist  in  the  Experiment  Station. 

s  Meteorologist,  United  States  Weather  Bureau,  in  charge  Fruit  Frost  Service. 


4  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

THE    ECONOMIC    IMPORTANCE    OF    FROST   PROTECTION 
IN    CALIFORNIA 

Although  California  is  favored  with  a  climate  which  is  essentially 
sub-tropical,  winter  and  spring  temperatures  sufficiently  low  to  cause 
damage  to  fruit  crops  are  occasionally  experienced.  All  weather 
records  from  the  earliest  indicate  that  such  temperatures  occur  at 
periodic  intervals. 

When  the  fruit  producing  industry  was  small  and  the  value  of 
the  crops  commensurably  low,  the  losses  caused  were  not  of  great 
magnitude.  With  the  rapid  extension  in  fruit  culture  which  has 
characterized  the  past  three  decades  and  the  great  increase  in  the 
value  of  the  crops  produced,  losses  from  frost  damage  have  become 
exceedingly  important,  amounting  to  millions  of  dollars  during  the 
past  five  years.  Few  if  any  localities  are  entirely  free  from  the 
danger  of  low  winter  and  spring  temperatures  and  with  minor 
exceptions  every  fruit  crop  of  commercial  importance  is  subject  to 
occasional  loss  from  this  cause. 

The  greatest  and  most  frequent  losses  occur  with  the  sub-tropical 
fruits,  on  account  of  their  greater  susceptibility  to  frost  damage. 
This  is  particularly  the  case  with  the  ever-green  sub-tropicals  such  as 
the  Citrus  fruits  and  the  avocado,  which  never  enter  a  condition  of 
complete  dormancy  and  which  normally  mature  their  crops  during 
the  winter  or  spring  months.  With  this  class  of  fruits,  therefore, 
losses  may  arise  not  only  from  the  destruction  of  a  part  or  all  of 
the  crop  but  from  the  killing  of  a  part  of  the  fruit-bearing  wood 
as  well.  With  mature  trees,  the  damage  to  the  fruit-bearing  wood 
may  be  sufficient  to  delay  fruit  production  for  several  years,  and 
with  young  trees  may  so  impair  their  usefulness  as  to  render  them 
practically  worthless,  sometimes  even  resulting  in  death. 

With  the  deciduous  fruits,  the  principal  damage  occurs  in  the 
late  winter  or  early  spring  months  and  usually  consists  only  in  the 
partial  or  entire  destruction  of  the  flowers  or  young  fruits,  with  a 
consequent  reduction  or  entire  failure  of  the  crop  for  the  season 
following  the  frost. 

In  the  Citrus  fruit  industry  at  least,  the  importance  of  losses  from 
low  winter  temperatures  is  widely  recognized  and  the  protection  of 
the  orchards,  as  much  as  practicable,  against  such  losses,  is  now 
generally  admitted  to  be  one  of  the  essentials  of  success.  Since  there 
are  practically  no  frostless  areas  in  the  state,  there  is  every  reason 
to  believe  that  orchard  heating  may  also  be  regarded  as  a  necessary 


BULL.  398]  ORCHARD   HEATING   IN    CALIFORNIA-  5 

factor  in  the  permanent  and  successful  culture  of  many  of  the 
other  sub-tropical  fruits  grown  in  California. 

Character  of  the  Losses  Sustained.  The  losses  resulting  from 
frost  damage  affect  in  varying  degree  the  individual  grower,  the  com- 
munity, the  fruit-growing  industry,  and  the  consuming  public. 

Direct  losses  suffered  by  the  growers  consist  in  the  partial  or 
entire  destruction  of  the  crop  for  the  current  season  and,  in  the  case 
of  the  ever-green  sub-tropicals,  reduction  in  crop  for  one  or  more 
seasons  afterward,  a  fact  which  is  often  underestimated. 

The  importance  of  frost  injury  as  a  factor  in  influencing  yield 
the  season  following  a  frost  is  shown  in  Table  1.  Estimated*  yields 
for  1921-22,  the  winter  of  a  severe  frost,  were  compared  with  actual 
yields  for  1922-23,  a  winter  during  which  no  frost  occurred,  from 
seven  packing  houses  in  one  district.  Three  of  these  houses  pack 
fruit  from  orchards  where  very  little  heating  is  done  and  the  other 
four  serve  orchards  in  colder  locations  most  of  which  have  been 
adequately  protected  by  orchard  heating.  The  table  shows  a  general 
increase  in  yield  from  heated  orchards  as  "a  result  of  protecting  the 
trees  themselves  from  injury  and  a  decrease  from  those  not  heated. 

TABLE  1 

Yield  for  1922-23,  the  Year  Following  a  Frost,  from  the  Orchards  Served 

by  Seven  Packing  Houses  in  One  District 

The  estimated  yield  for  the  frost  year  1921-22  is  taken  as  100  per  cent. 

1922-23  1921-22 

Per  cent  Per  cent 

House  No.  1,  orchards  not  heated  71  100 

House  No.  2,  orchards  not  heated  72  100 

House  No.  3,  orchards  not  heated  89  100 

House  No.  4,  most  orchards  heated  104  100 

House  No.  5,  most  orchards  heated  105  100 

House  No.  6,  most  orchards  heated  113  100 

House  No.  7,  most  orchards  heated  134  100 

These  increases  reported  for  houses  4,  5,  6  and  7  are  in  addition 
to  the  fruit  saved  in  1921-22  when  the  crop  in  the  unheated  orchards 
was  practically  all  lost.  If  frost  losses  were  uniformly  distributed 
among  all  the  fruit  growers  there  would  be  little  complaint  because  of 
the  well-known  fact  that  after  a  frost  the  portion  of  the  crop  which 
escapes  damage  usually  brings  a  net  return  to  the  industry  equal 
to  or  sometimes  exceeding  that  normally  received  for  the  entire  crop. 

*  Actual  yield  records  for  1921-22  are  not  available  because  of  extensive 
damage  to  fruit  by  the  frost  of  that  year.  The  estimated  yields  are  quite 
accurate  because  the  frost  occurred  late  in  the  season  after  the  fruit  had  grown 
to  full  size. 


6  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Thus  it  is  not  at  all  uncommon  for  growers  who  have  recently 
installed  orchard  heating:  equipment  to  reap  a  return  from  the  saving 
of  one  crop  sufficient  to  pay  for  the  cost  of  the  equipment  and  the 
overhead  costs  of  its  maintenance  for  a  period  of  years. 

Since  it  seems  probable  that  the  enhanced  value  of  the  fruit  saved 
during  a  cold  winter  will  be  reduced  more  or  less  in  proportion  to 
the  extent  to  which  orchard  heating  is  adopted,  some  growers  have 
questioned  the  advisability  of  educational  efforts  designed  to  stimulate 
interest  in  orchard  heating.  It  is  their  belief  that  when  orchard 
heating  becomes  sufficiently  widespread  to  reduce  markedly  the 
variations  in  crop  production,  the  profits  returned  from  the  use  of 
heaters  will  be  greatly  reduced.  But  even  if  orchard  heating  should 
become  very  general,  which  is  highly  improbable,  the  use  of  orchard 
heaters  will  undoubtedly  continue  to  be  profitable  for  reasons  which 
will  be  given  later. 

In  addition  to  the  losses  from  crop  and  tree  damage  which  are 
immediate  and  therefore  most  in  evidence,  there  are  losses  of  an 
indirect  character,  in-  many  cases  equally  serious  in  their  effects  on 
the  community,  and  spread  over  so  long  a  period  that  they  are 
frequently  overlooked  or  at  least  under-estimated.  The  indirect  losses 
arising  from  frost  damage,  so  far  as  the  individual  grower  is  con- 
cerned, in  most  cases  result  from  reduction  in  the  working  capital 
available  for  upkeep  of  the  orchards.  The  loss  of  one  or  two  crops 
has  been  responsible  in  many  instances  for  inability  on  the  part  of 
the  grower  to  properly  fertilize  the  orchard  or  to  provide  for  adequate 
insect  pest  control  measures,  both  of  which  are  necessary  to  continued 
profitable  fruit  production.  The  effects  of  neglect  arising  from  losses 
through  frost  in  some  cases  have  persisted  for  years,  and  in  certain 
communities  have  been  responsible  for  a  marked  decline  in  orchard 
values. 

The  losses  to  the  community  are  no  less  serious  than  the  losses 
to  the  grower.  They  include  decreased  employment  of  persons 
engaged  in  fruit  handling  operations,  as  well  as  reduction  of  the 
general  community  income.  In  fact  the  serious  losses  sustained  by 
certain  communities  as  a  result  of  repeated  frost  damage  have  led  to 
the  recognition  of  orchard  heating  as  a  community  problem. 
Cooperation  of  the  business  men  has  enabled  the  growers  to  finance 
the  purchase  of  orchard  heating  equipment  and  thus  to  afford 
protection  which  would  otherwise  have  been  unobtainable. 

In  the  case  of  the  Citrus  fruits  the  sudden  loss  of  a  crop  ready 
for  market  results  in  serious  losses  to  all  individuals,  agencies  and 
organizations  concerned  in  packing,  shipping  and  marketing  the  crop. 


Bull.  398]  orchard  heating  in  California  7 

The  frost  damage  occurs  after  arrangements  have  been  completed  for 
the  purchase  of  packing  materials,  appropriations  have  been  made 
for  national  advertising,  and  schedules  for  moving  the  crop  have 
been  worked  out  with  the  railroads.  The  necessary  cancellations  of 
orders  for  material,  and  services  result  in  losses  of  a  widespread 
character. 

•Although  the  maintenance  of  an  absolutely  constant  source  of 
supply  of  any  agricultural  product  is  manifestly  impossible,  the 
adoption  of  orchard  heating,  where  practicable,  furnishes  an  impor- 
tant means  of  reducing  variation.  The  ability  at  all  times  to  supply 
the  market  demand  for  their  product  becomes  a  necessity  to  cooper- 
ative selling  organizations  in  developing  and  holding  markets  against 
competition.  The  California  Citrus  fruit  marketing  agencies  have  on 
many  occasions  been  unable,  because  of  frost  damage,  to  supply 
sufficient  fruit  to  meet  the  market  demand,  and  the  efforts  of  years 
expended  in  developing  and  holding  certain  markets  have  thus  been 
undone  in  a  few  months. 

While  the  high  prices  which  frequently  folloAV  a  frost  may  be 
beneficial  to  the  growers  who  have  sound  fruit  to  ship,  the  growers 
with  little  or  no  fruit  to  market  lose  heavily.  Excessive  prices  cause 
consumers  to  turn  their  attention  to  other  products  and  the  marketing 
problems  of  the  industry  are  rendered  more  difficult  for  the  years  in 
which  no  frost  damage  occurs. 

The  Present  Status  of  Orchard  Heating.  It  is  not  strange,  there- 
fore, that  the  prevention  of  losses  of  fruit  crops  occasioned  by 
damaging  winter  and  spring  temperatures  should  have  come  to  be 
regarded  as  among  the  most  important  problems  confronting  Cali- 
fornia fruit  growers.  Nor  is  it  surprising  that  orchard  heating,  an 
ancient  practice  in  fruit  culture,  should  have  reached  its  highest 
development  and  most  extensive  utilization  in  California.  As  an 
evidence  of  the  rapid  extension  of  this  practice  in  California  fruit 
orchards  in  recent  years  may  be  cited  the  fact  that,  so  far  as  records 
are  available,  in  the  past  fifteen  years  approximately  1,500,000 
orchard  heaters  have  been  purchased  by  California  Citrus  growers, 
the  greater  part  of  which  are  now  in  use.  The  unusual  frequency 
of  damaging  winter  temperatures  during  the  past  three  years  has 
given  a  decided  stimulus  to  the  installation  of  orchard  heating  equip- 
ment. Reports  indicate  that  during  the  present  season  approximately 
600,000  heaters  have  been  purchased  by  California  Citrus  fruit 
growers.  The  present  investment  in  this  state  in  orchard  heating 
equipment  is  certainly  not  less  than  $2,500,000  and  the  area  under 
frost  protection  is  approximately  thirty  thousand  acres.     There  is 


8  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

reason  to  believe  that  in  the  citrus  fruit  industry  alone  there  is  need 
for  approximately  5,000,000  orchard  heaters  which  would  provide 
for  more  than  trebling  the  acreage  under  protection  at  the  present 
time. 


WHEN    DOES    IT    BECOME    ADVISABLE    TO    INSTALL   ORCHARD 
HEATING  EQUIPMENT? 

The  question  of  when  or  under  what  conditions  it  becomes 
advisable  for  the  grower  to  install  orchard  heating  equipment  is 
difficult  if  not  impossible  to  answer  satisfactorily,  since  there  are  so 
many  variable  factors  such  as  differences  in  topography  and  air 
drainage  concerned.  In  the  last  analysis  this  question  must  be 
answered  by  each  grower  for  himself.  There  are,  however,  certain 
factors  entering  into  the  decision  as  to  whether  or  not  to  install 
heating  equipment  which  are  capable  of  determination  with  some 
degree  of  accuracy. 

There  is  every  reason  to  believe  that  over  wide  areas  in  the  fruit 
producing  sections  of  the  state  the  occurrence  of  temperatures 
occasioning  severe  damage  is  so  infrequent  that  the  savings  effected 
by  the  use  of  heaters  would  not  in  the  long  run  equal  the  costs  of 
installing  and  operating  the  heating  equipment.  It  is  also  probable 
that  there  are  certain  localities  planted  to  fruits  where  frost  damage  is 
so  extensive  and  so  frequent  in  occurrence  that  over  a  period  of  years 
the  cost  of  heating  would  exceed  the  value  of  the  crops  saved.  Orchards 
so  situated  should  be  top-worked  to  varieties  either  more  resistant 
to  frost  damage,  or,  on  account  of  later  blooming  or  earlier  maturity 
of  the  fruit,  offering  fewer  hazards.  Unless  this  is  possible  it  is 
doubtful  whether  such  orchards  should  continue  to  be  maintained 
and  in  most  cases  they  must  eventually  be  abandoned  or  replaced  by 
other  crops. 

It  is  unsafe  to  make  a  decision  concerning  the  advisability  of 
installing  orchard  heaters  from  the  experience  of  only  one  season. 
The  question  should  be  given  careful  study  and  a  decision  arrived 
at  only  when  it  is  apparent  from  the  experience  of  a  number  of  years 
that  orchard  heating  is  not  only  necessary  to  obtain  satisfactory  crops 
but  that  it  will  probably  pay  returns  on  the  investment  in  capital 
and  labor  involved  in  its  installation  and  operation.  It  is  the  purpose 
of  the  present  discussion  to  emphasize  the  fact  that  orchard  heating 
should  be  undertaken  only  where  it  is  likely  to  pay,  and  to  indicate 
a  method  of  determining  when  orchard  heating  is  advisable. 


BULL.  39S]  ORCHARD   HEATING   IN    CALIFORNIA  9 

The  primary  factors  which  should  determine  the  advisability  of 
orchard  heating  are  the  overhead  and  operating  costs  involved,  and 
the  probable  savings  which  may  result.  The  overhead  costs  of  orchard 
heating  can  be  determined  with  some  degree  of  accuracy  from  the 
extensive  experience  at  hand.  Operating  costs  can  be  estimated  with 
a  fair  degree  of  accuracy  providing  the  average  number  of  hours  of 
heating  per  year  required  to  save  the  crop  is  known.  In  the  majority 
of  cases  it  will  be  necessary  to  make  an  estimate  on  this  point  since 
in  many  districts  comprehensive  temperature  data  are  lacking. 
Estimates  should  be  based  on  available  data  and  in  all  cases  should 
be  liberal. 

The  probable  savings  from  heating  are  difficult  to  compute  since 
they  are  determined  by  the  production  per  acre  which  may  be 
expected  under  the  methods  of  management  in  use  and  the  average 
price  which  may  be  expected  for  the  fruit  saved.  It  is  necessary, 
therefore,  to  estimate  the  production  per  acre  that  may  be  expected 
under  competent  management  and  the  percentage  of  the  crop  that 
is  likely  to  be  lost  over  a  ten-year  period  as  a  result  of  frost.  As 
previously  indicated,  the  average  price  received  for  fruit  saved  during 
a  frosty  period  is  generally  somewhat  higher  than  the  average  received 
during  normal  seasons.  It  is  believed,  however,  that  the  safest  proce- 
dure is  to  use  average  prices  for  the  entire  period  rather  than  to  base 
the  calculations  on  average  prices  received  in  years  of  frost  damage. 

The  factors  of  production  per  acre  and  average  price  received  for 
the  fruit  are  of  great  importance  in  determining  the  probable  profits 
from  orchard  heating.  With  a  given  cost  of  heating  over  a  ten-year 
period,  it  may  prove  to  be  a  profitable  investment  if  large  crops  are 
produced,  even  though  prices  received  for  the  fruit  may  not  be 
abnormally  high.  Heating  may  also  prove  profitable  in  districts  or 
with  varieties  where  on  account  of  local  conditions  the  price  received 
for  the  fruit  is  above  the  average  even  though  the  production  per 
acre  is  not  unusually  high.  It  is  clear  that  where  both  average  yields 
and  prices  are  low  heating  is  likely  to  result  in  a  loss.  These  general 
relations  can  be  expressed  in  the  statement  that  the  average  value 
of  the  crop  is  a  factor  of  the  greatest  importance  in  determining  the 
advisability  of  orchard  heating.  In  this  connection  a  fundamental 
difference  between  orchard  heating  and  frost  insurance  may  be 
emphasized,  namely  that  in  the  former  the  cost  is  relatively  constant 
for  any  given  area  and  is  independent  of  the  value  of  the  crop, 
whereas  in  the  latter  the  premiums  are  proportional  to  the  value  of 
the  crop. 


10  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

To  illustrate  the  method  of  determining  the  advisability  of 
orchard  heating  several  hypothetical  cases  of  heating  oranges  will  be 
considered  in  tables  2  to  8.  The  figures  used  are  on  a  cost  per  acre 
basis  and  are  only  approximate.  Further  details  concerning  actual 
heating  costs  will  be  found  in  another  section. 

In  making  the  computations  the  cost  of  the  equipment  is  placed 
at  $150  per  acre  exclusive  of  fuel  in  storage.  A  10  per  cent  deprecia- 
tion rate  is  allowed  and  interest  at  3  per  cent  which  will  give  an 
actual  return  of  6  per  cent  on  an  investment  depreciating  at  the  rate 
of  10  per  cent  per  annum.  Interest  on  fuel  in  storage  is  charged  at 
6  per  cent.  Typical  operating  expenses  are  $1.00  per  acre  per  hour 
for  fuel  and  labor,  $2.00  per  acre  for  refilling,  $2.00  per  acre  for 
placing  heaters  in  the  orchard,  including  the  initial  filling,  and  $2.00 
per  acre  for  emptying  and  removing  heaters  from  the  orchard.  On 
this  basis  the  annual  fixed  charges  per  acre  will  be  those  shown  in 
table  2. 

TABLE  2 
Fixed  Charges  (Whether  Heaters  are  Lighted  or  Not) 

Depreciation    on   equipment $15.00 

Interest  on  investment   (including  oil)  6.00 

Placing  and  filling  heaters 2.00 

Emptying  and  taking  up  heaters 2.00 

Total $25.00 

In   the   hypothetical   cases   considered   the    following   values    for 
production,  price,  value  and  cost  are  assumed : 
Average  Production:   200  packed  boxes  per  acre. 
Average  Price  of  Fruit:  $2.25  a  packed  box  net  to  the  grower. 
Average  Value  of  Crop:   $450  per  acre  net  to  the  grower. 
Average  Cost  of  Production:    $240  per  acre  (excluding  heating). 

Case  I 

table  3 

Orchard  Heating  Costs  per  Acre  for  a  Ten  Year  Period 

Oranges 

Overhead  charge   $250.00 

Operation  costs 

3  years  not  necessary  to  light  up  - 

1  year     (15  nights,  90  hours,  8  refillings)  106.00 

2  years    (10  nights,  50  hours,  5  refillings) 120.00 

4  years   (    5  nights,  25  hours,  3  refillings)  124.00 

10  Total $600.00 

Average  number  of  hour  heating  required,  29. 


Bull.  308]                      ORCHARD   HEATING    IN    CALIFORNIA  11 

TABLE  4 
Savings  Made  by    Heating  Operations 

Per  cent  Value  of  crop  saved, 

of  crop  per  acre 

3  years    none  

1  year    100  $450.00 

2  years    50  450.00 

4  years 25  450.00 


10  years                                                                                 Total,  $1,350.00 
Cost  of  heating 600.00 


Saving  per  acre  for  10  year  period $750.00 

Average  saving  per  acre  per  year,  $75.00,  on  investment  of  $60.00. 

TABLE  5 
Summary  of  Returns  per  Acre  for  Ten  Year  Period 
With  Heating  Without  Heating 

Income  (2,000  packed  boxes  Income  (1,400  packed  boxes 

@  $2.25)  $4,500.00  @    $2.25) $3,150.00 

Production  costs  3,000.00  Production  costs 2,400.00 


Profit  $1,500.00  Profit $750.00 

Or   at   the   rate   of   $150.00   per   acre.  Or  at  the  rate  of  $75.00  per  acre. 

It  is  clearly  apparent  in  this  case  that  the  installation  of  heating 
equipment  paid  a  substantial  return  on  the  investment  and  that 
through  the  addition  of  heating  to  the  orchard  management  program 
the  rate  of  return  from  the  orchard  was  doubled. 


Case  II 

Data  used  in  computations  same  as  in  Case  I  with  exception  of 
orchard  heating  costs. 

TABLE  6 

Orchard  Heating  Costs  per  Acre  for  a  Ten  Year  Period 

Oranges 

Overhead  charge  $250.00 

Operation  costs 

10  years  (average  8  nights,  80  hours,  8  refillings)  960.00 

Total    $1,210.00 

Average  number  of  hours  of  heating  required,  80. 


12 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


TABLE  7 
Savings  Made  by  Heating  Operations 

Per  cent  Value  of  crop  saved, 

of  crop  per  acre 

4  years    100  $1,800.00 

4  years    50  900.00 

2  years    25  225.00 

10  years                                                                                    Total,  $2,925.00 
Cost  of  heating 1,210.00 

Saving  per  acre  for  period $1,715.00 

Average  saving  per  acre  per  year,  $171.50,  on  investment  of  $121.00. 


TABLE  8 
Summary  of  Eeturns  per  Acre  for  Ten  Year  Period 
With  Heating  Without  Heating 

Income  (2,000  packed  boxes  Income    (700  packed  boxes 

@  $2.25)  $4,500.00  @    $2.25) $1,575.00 

Production  costs  3,610.00  Production  costs  2,400.00 


Profit  $890.00 

Or  at  rate  of  $89.00  per  year. 


Loss  $825.00 

Or  at  rate  of  $82.50  per  year. 


It  will  be  observed  that  in  this  case  the  addition  of  heating  to 
the  orchard  management  program  has  changed  the  situation  from 
one  of  loss  to  profit.  The  profit,  however,  is  relatively  small  although 
the  saving  resulting  from  heating  is  large.  Undoubtedly  there  are 
many  Citrus  orchards  where  this  situation  obtains. 

Some  relations  between  average  price  received  for  the  fruit  and 
amount  of  fruit  produced  (per  acre)  and  the  savings  effected  by 
orchard  heating  will  be  noted  in  tables  9  and  10. 

Although  in  the  case  of  deciduous  fruits  both  the  cost  of  heating 
and  the  savings  effected  will  be  much  lower  than  those  used  in  the 
hypothetical  cases  above  discussed,  the  methods  used  in  determining 
the  advisability  of  installing  orchard  heating  equipment  are 
applicable. 


Bull.  308] 


ORCHARD    HEATING    IN    CALIFORNIA 


13 


TABLE  9 

Eelation  Between  Price  Received  for  Fruit  and  Returns  per  Acre  made 
.from  Orchard  Heating 

All  data  used  same  as  in  Case  1  with  exception  of  price  received  for  fruit. 


Ten-year 

production 

packed 

boxes  of 

75  lbs 


Ten-year 

cost  of 
production 


Net  profits  over  10-year  period 


2  cents 
per  pound 


3  cents 
per  pound 


4  cents 
per  pound 


5  cents 
per  pound 


Orchard  heated. 


Orchard  not 
heated 


Difference  due  to 
heating 


2000 


1400 


600 


$3000.00 


2400.00 


600.00 


—Cost 
and  value 
same  (no 

profit) 

$300.00 
loss 

$300.00 
saved 


$1500.00 


750.00 


750.00 


$3000.00 


1800.00 


1200.00 


$4500.00 


2850.00 


1650.00 


TABLE  10 

Relation  Between  Production  per  Acre  and  Returns  made  per  Acre  by 

Orchard  Heating 

All  data  used  same  as  in  Case  1  with  exception  of  production  per  acre. 


Ten-year 

cost  of 
production 

Netjprofit  over  10-year  period[from  varying  yields 

100* 

200* 

300* 

400* 

500* 

Orchard  heated... 

Orchard  not 
heated 

$3000.00 

$2400.00 

600.00 

$750.00 
loss 

$825.00 
loss 

75.00 
saving 
in  loss 

$1500.00 
750.00 
750.00 

$3750.00 
2325.00 
1425.00 

$6000.00 
3900.00 
2100.00 

$8250.00 
5475.00 

Difference  due  to 
heating 

2775.00 

Tacked  boxes  of  75  pounds. 


14  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

PHYSICAL    PRINCIPLES    OF    FROST    FORMATION    AND 
FROST    PROTECTION 

Definition  of  Terms.  Frost  may  be  purely  local  or  of  general 
occurrence  and  may  be  either  ' '  white  "  or  ' ' black ' '  depending  on  atmo- 
spheric conditions,     The  terms  for  frost  may  be  denned  as  follows : 

1.  Frost.     The  occurrence  of  any  temperature  below  32°  F. 

2.  White  Frost.     Frost  accompanied  by  a   deposit  of  white  ice 

crystals  on  exposed  surfaces. 

3.  Black  Frost.    Frost  unaccompanied  by  such  a  deposit. 

If  local  cooling  by  radiation  causes  the  frost  it  is  spoken  of  as  a 
local  frost  or  as  a  radiation  frost. 

The  widespread  occurrence  of  temperatures  below  32°  F.  accom- 
panied by  or  following  a  period  of  heavy  winds  and  an  influx  of  cold 
air  from  distant  areas  is  called  a  general  frost,  or  technically  a 
freeze.  All  frosts  or  freezes  will  be  "white"  if  the  air  is  sufficiently 
moist  and  "black"  if  the  air  is  very  dry.  Frost,  i.e.  temperatures 
of  32°  F.  or  less,  may  occur  on  the  ground  or  exposed  objects  as  a  result 
of  rapid  cooling  by  radiation  even  though  the  temperature  of  the 
air  in  the  vicinity  of  the  exposed  surfaces  may  be  above  32°  F. 

How  Frost  Occurs.  Many  persons  apparently  believe  that  frost 
and  dew  "fall"  in  the  same  way  that  rain  falls  as  evidenced  by  the 
fact  that  a  number  of  inventions  have  been  patented,  which  were 
designed  with  the  idea  of  preventing  the  frost  from  "falling"  on  the 
trees.  Deposits  of  frost  or  dew  are  formed  directly  on  the  ground 
or  other  exposed  objects  as  a  result  of  local  cooling. 

In  order  to  understand  the  underlying  principles  of  frost 
protection,  it  is  necessary  to  know  something  of  the  methods  by  which 
the  ground  surface  and  the  air  near  the  ground  cool  during  the 
night.  The  heat  from  the  sun  comes  to  the  earth  in  the  form  of  waves, 
a  method  of  heat  transfer  which  is  known  as  radiation.  Heat  is  also 
lost  to  the  intensely  cold  upper  limits  of  the  atmosphere  by  this  same 
process  of  radiation.  The  earth  loses  heat  by  radiation  continuously, 
both  day  and  night,  but  during  the  day  the  amount  of  heat  received 
from  the  sun  is  much  greater  than  that  radiated  into  space,  and  the 
temperature  rises.  Radiant  heat  passes  through  clear  dry  air  without 
much  heating  of  the  air  itself.  Air  is  warmed  much  more  effectually 
by  contact  with  a  warmer  body,  that  is  by  conduction  of  heat  from 
the  warm  body  to  the  cooler  air  in  contact  with  it.  The  transfer  of 
heat  by  radiation  occurs  at  very  high  speed  (the  velocity  of  light) 
while  conduction  is  relatively  a  very  slow  process. 


Bull.  398]  orchard  heating  in  California  15 

During-  a  clear,  calm  day  the  radiant  heat  from  the  sun  heats  the 
ground  surface  until  its  temperature  is  higher  than  thai  of  the  air 
in  contact  with  it.  As  soon  as  this  occurs,  heat  is  slowly  conducted 
from  the  ground  into  the  surface  layer  of  air,  which  soon  becomes 
warmer  than  the  air  at  higher  elevations.  Warm  air  is  lighter  (less 
dense)  than  cold  air,  and  as  soon  as  a  small  body  of  air  in  contact 
with  the  ground  becomes  warmer  than  that  above  or  surrounding  it, 
it  is  forced  upward  and  is  replaced  by  colder  air.  A  circulation  is 
thus  established,  in  wmich  cool  upper  air  is  progressively  brought  into 
contact  with  the  w^arm  ground,  heated  by  conduction,  and  then  forced 
upward  to  make  room  for  more  cool  air.  By  sunset  the  air  to  a 
height  of  300  to  1000  feet  has  been  warmed  to  some  extent.  The 
fact  that  heating  the  air  reduces  its  density,  operates  to  prevent  the 
heat  received  from  the  sun  being  concentrated  in  the  surface  layer 
of  air  alone,  and  causes  distribution  of  the  heat  through  a  layer  of 
considerable  thickness.  The  transference  of  heat  as  described  above 
from  one  portion  of  a  liquid  or  gaseous  medium  such  as  the  air  to 
another  through  the  circulation  of  portions  of  the  medium  is  called 
convection. 

After  sunset,  no  heat  is  received  from  the  sun  to  make  up  for  that 
lost  by  radiation  from  the  ground  to  the  sky,  and  the  ground  soon 
becomes  colder  than  the  layer  of  air  in  contact  with  it.  Heat  is 
conducted  from  the  air  to  the  ground  and  the  surface  layer  of  air 
soon  becomes  colder  than  the  air  a  few  feet  above.  In  this  case, 
however,  the  surface  air  becomes  relatively  heavier  as  it  continues 
to  cool  during  the  night,  so  that  the  tendency  is  for  the  same  air  to 
remain  in  contact  with  the  ground  all  night.  Since  air  conducts 
heat  very  slowly,  atmospheric  cooling  does  not  extend  to  great  heights 
as  a  result  of  which  the  temperature  of  the  air  300  feet  above  the 
ground  changes  but  little  during  the  night.  Thus  over  a  level  plain 
on  a  clear,  calm  night  there  is  a  relatively  thin  layer  of  cold  air  near 
the  ground,  with  an  increase  in  temperature  up  to  an  altitude  of 
between  300  and  800  feet.  This  phenomenon,  wmich  is  known  as 
temperature  inversion,  is  illustrated  in  figure  1,  wmich  shows  con- 
tinuous records  of  the  temperatures  from  4  P.M.  to  9  A.M.  at  the  base 
and  at  different  heights  above  the  base  of  a  steep  hillside.  Note  the 
great  differences  in  temperature  that  sometimes  develop  on  a  clear, 
still  night.  Although  the  temperature  at  the  base  was  low  enough 
to  cause  considerable  damage  to  fruit,  the  lowest  temperature  225  feet 
above  the  base  was  only  51°F.  Note  that  the  duration  of  the  lowest 
temperature  wTas  much  shorter  on  the  hillside  than  at  the  base. 


16 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


When  the  loss  of  heat  by  radiation  from  the  ground  has  been 
sufficient  to  cool  it  to  32°  F.  or  lower,  a  true  radiation  frost  occurs. 
Under  radiation  frost  conditions  the  force  of  gravity  tends  to  cause 
the  thin  surface  layer  of  cold  air  to  move  down  slopes  and  to  gather 
in  depressions.  On  a  frosty  night  this  movement  of  air,  which  is 
known  as  air  drainage,  operates  to  create  a  difference  in  temperature 
between  the  hillsides  and  the  flat  valley  floors.  The  air  cooled  by 
contact  with  the  cooler  ground  constantly  drains  away  from  hillside 
locations  and  is  replaced  with  warmer  air  from  the  same  or  slightly 


64° 

8P.M.             4PM.                6P.M.               SPM           /OPM              MDT              2AM               4A.M.              6/JM               SAM. 

55" 
50" 
4f 

1  '\ 

^Jjfv 

/, 

\\ 

\ 

f        V  * 

r\ 

i  A 

•••% 

....  :"'":.;" '• 

-.!•; 

j? 

i 

\  *"i 
\   i 
\  \ 

V  V 

■  V 

\ 

n 

•j 

l\ 
1 
il 

I 

/ 1 

40" 

remperafure  B25  feet  above  base  station. 
remp era  tare  JO  feef  above  base  s tot/on. 
"e/nperature  P5  fee/  above  £>ase  station, 
'empero/vre  a/  6ase  station 

\v 

^~N 

3S° 

i*»*A 

s— \. 

V 

^.r- 

-y-N 

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/ 

j 

/ 

Fig.  1. — Temperature  records  taken  at  different  altitudes  on  a  frosty  night. 


higher  elevations.  Over  level  areas  the  cold  air  tends  to  remain  more 
nearly  stationary,  growing"  colder  and  colder  as  the  night  progresses. 
Cold  air  from  surrounding  slightly  higher  areas  flowing  into  depres- 
sions that  either  have  no  outlets,  or  outlets  too  small  to  permit  the 
draining  away  of  the  cold  air  as  fast  as  it  flows  in  creates  pools  of 
still  and  usually  relatively  very  cold  air  which  are  sometimes  called 
"frost  pockets." 

Difference  Between  Frosts  and  Freezes.  As  has  just  been  noted 
true  frosts  result  from  excessive  local  cooling  by  radiation  and  occur 
on  relatively  calm  nights.  There  are  occasional  periods  of  extremely 
cold  weather,  however,  accompanied  or  preceded  by  high  winds  in 


*  Young,  Floyd  D.     Frost  and  the  prevention  of  damage  by  it. 
Farmers'  Bull.  1096,  1922. 


U.  S.  D.  A. 


BULL.  398]  ORCHARD   HEATING   IN    CALIFORNIA  17 

which  a  general  cooling  of  the  air  takes  place  to  great  heights  on 
account  of  the  influx  of  large  masses  of  cold  air  from  the  North  or 
Northeast.  Such  cold  periods  are  designated  as  freezes  if  the  tem- 
perature drops  below  32°  F.  A  true  freeze  of  this  kind  rarely  occurs 
in  southern  California.  If  a  freeze  as  just  described  is  followed  by 
a  dying  down  of  the  wind  and  accompanied  by  atmospheric  conditions 
permitting  rapid  loss  of  heat  by  radiation  the  result  is  a  general  low- 
temperature  with  only  a  slight  temperature  inversion;  a  condition 
very  difficult  to  control  with  orchard  heaters. 

How  Temperature  Inversion  Makes  Heating  Possible.  The  differ- 
ence in  temperature  between  the  air  near  the  ground  and  that  at 
higher  levels  on  a  frosty  night,  as  illustrated  above,  is  what  makes 
orchard  heating  practicable.  If  the  atmosphere  wrere  uniformly  cold 
up  to  great  heights,  the  air  heated  by  the  fires  in  the  orchard  would 
rise  rapidly  above  the  orchard  without  materially  benefiting  the  trees 
or  fruit.  As  a  matter  of  fact,  the  warmed  air  from  the  heaters  rises, 
cooling  at  the  same  time,  until  it  reaches  the  height  where  its  tem- 
perature is  the  same  as  that  of  the  surrounding  air.  As  the  hot 
gases  leave  the  fires,  they  mix  rapidly  with  the  surrounding  colder 
air,  so  that  the  resulting  temperature  of  the  whole  mass  is  not  very 
high.  So  long  as  the  mixture  of  gases  rising  from  the  fires  is  warmer 
than  the  surrounding  air  it  will  continue  to  rise;  but  as  soon  as  it 
reaches  an  elevation  where  its  temperature  is  the  same  as  that  of  the 
surrounding  air  it  will  stop  and  remain  stationary. 

As  an  example  of  a  typical  case,  let  us  assume  that  the  air  in  the 
orchard  at  a  height  of  five  feet  from  the  ground  has  a  temperature 
of  24°  F.  and  that  the  air  forty  feet  above  the  ground  has  a  tem- 
perature of  32°  F.  When  the  orchard  heaters  are  lighted,  the 
relatively  small  quantities  of  hot  gases  produced  are  mixed  with  air 
which  has  a  temperature  of  24°  F.  If  the  mixture  which  results  has 
a  temperature  of  32°  F.  its  specific  gravity  will  be  lower  and  it  will 
rise  until  the  40-foot  level  is  reached.  At  that  height  it  will  stop 
rising  because  the  surrounding  air  is  of  the  same  temperature  and 
specific  gravity.  Successive  additions  of  the  heated  gases  will  come 
to  a  stop  below  the  first  layer  and  the  process  continues  until  the 
temperature  of  the  air  down  to  the  ground  has  been  raised  to  32°  F. 
The  temperature  inversion  will  then  have  disappeared,  and  the  tem- 
perature in  the  orchard  at  the  five-foot  level  will  have  been  raised  eight 
degrees,  from  24°  F.  to  32°  F.  It  is  plain  that  in  this  case  the  heat  pro- 
duced by  the  orchard  heaters  has  been  expended  in  heating  the  air 
within  40  feet  of  the  ground,  and  has  not  been  wasted  in  a  futile 


18  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

attempt  to  "warm  up  all  out-of-doors."  The  stratum  of  warm  air 
above  the  orchard  acts  as  a  ceiling  to  prevent  the  escape  of  the  heated 
air. 

The  example  given  above  describes  the  effect  of  orchard  heating 
under  ideal  conditions,  with  a  perfectly  calm  atmosphere  in  and 
immediately  above  the  orchard.  Occasionally  these  conditions  occur, 
but  on  most  frosty  nights  there  is  a  light  drift  of  air,  which  carries 
heat  out  of  the  heated  orchard,  and  affords  protection  to  a  few  row7s 
of  trees  in  the  immediately  adjoining  orchard  on  the  leeward.  For 
this  reason  it  is  more  difficult  to  raise  the  temperature  when  only 
one  orchard  in  the  neighborhood  is  protected  with  heaters  than  when 
all  the  surrounding  orchards  are  thus  protected. 

The  amount  of  temperature  inversion  varies  greatly  on  different 
nights  and  in  different  localities.  It  is  mainly  determined  by  the 
range  in  temperature  from  afternoon  to  early  morning.  If  the 
afternoon  temperature  is  high  and  the  temperature  falls  to  the 
freezing  point  on  the  following  morning  the  inversion  in  temperature 
is  likely  to  be  great.  Since  the  thickness  of  the  layer  of  air  to  be 
heated,  in  order  to  obtain  a  given  rise  in  temperature  in  the  orchard, 
depends  on  the  amount  of  temperature  inversion,  it  follows  that  it 
is  easier  to  raise  the  temperature  following  a  warm  afternoon  than 
following  a  cold,  windy  day. 

Atmospheric  Conditions  Influencing  the  Occurrence  of  Frost.  It 
has  been  noted  previously  that  the  earth  cools  at  night  through  loss  of 
heat  by  radiation  from  the  ground.  The  more  rapid  this  loss  of  heat 
the  faster  is  the  rate  of  fall  in  temperature.  The  rate  of  radiation 
is  influenced  considerably  by  the  amount  of  water  vapor  in  the 
atmosphere.  The  greater  the  amount  of  water  vapor  in  the  atmos- 
phere the  lower  is  the  rate  of  radiation  loss,  and  conversely.  On  calm 
nights  the  temperature  falls  more  slowly  if  the  air  is  damp  than  if 
it  is  dry. 

The  amount  of  moisture  that  the  atmosphere  can  hold  depends 
upon  the  temperature.  As  air  is  warmed  its  moisture  holding  capacity 
is  increased ;  as  it  cools  its  capacity  for  moisture  is  decreased.  If  the 
air  contains  water  vapor  up  to  the  limit  of  its  capacity,  it  is  said  to 
be  " saturated."  If  the  temperature  of  saturated  air  is  reduced,  a 
portion  of  the  moisture  is  condensed.  This  is  what  occurs  when  dew 
or  white  frost  forms.  At  night  the  temperature  of  the  air  continues 
to  fall  until  mosture  is  deposited  on  exposed  surfaces.  The  tempera- 
ture at  which  dew  or  white  frost  begins  to  form  is  called  the  "dew 
point."     By  measuring  the  amount  of  moisture  in  the  air  at  any 


Bull.  398] 


ORCHARD    HEATING    IN    CALIFORNIA 


19 


time,  Ave  are  able  to  tell  its  dew  point ;  in  other  words,  we  can  know 
in  advance  at  what  temperature  dew  or  white  frost  will  begin  to  form 
if  the  cooling  of  the  air  continues. 

In  addition  to  decreasing  the  rate  at  which  heat  is  lost  by 
radiation  to  a  clear  sky  at  night,  the  moisture  in  the  air  has  another 
important  function  in  controlling  temperature  changes;  when  water 
vapor  changes  from  a  gaseous  to  a  liquid  state,  in  other  words,  when 
dew  or  white  frost  forms,  heat  is  released  in  the  process.  The  amount 
of  heat  released  is  proportional  to  the  amount  of  moisture  condensed. 


Vt 

7  PM 

8P*. 

9 

pn 

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Fig.  2. — Temperature  records  taken  on  two  different  nights  during  the  freeze 

of  January,  1922.* 

If  the  dew  point  is  high,  a  great  deal  of  dew  is  formed  before  the 
temperature  falls  to  the  freezing  point  and  a  large  amount  of  heat 
is  released,  which  will  materially  decrease  the  rate  of  fall  in  tem- 
perature. If  the  dew  point  is  low,  18°  F.,  for  example,  neither  dew  nor 
white  frost  will  form  until  the  ground  surface  and  objects  exposed 
to  the  sky  are  cooled  to  18°  F.  Under  such  conditions  the  temperature 
fall  will  be  rapid. 

Figure  2  shows  the  influence  of  the  amount  of  moisture  in  the 
air,  or  in  other  words,  of  the  dew  point,  upon  the  rate  of  temperature 
fall  under  radiation  frost  conditions.    The  upper  record  (dotted  line) 

*  Young,  Floyd  D.  Conditions  which  occasion  injurious  freezes  in  the  south. 
Calif.  Citrograph.     7:381.     1922. 


20  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

indicates  the  temperature  on  the  night  of  January  22-23,  1922, 
when  the  dew  point  was  29°  F.  The  lower  record  (solid  line)  shows 
the  temperature  on  the  night  of  January  19-20,  1922,  when  the  dew 
point  was  18°  F.  The  temperature,  in  degrees,  is  shown  at  the  left  of 
the  diagram,  and  the  time  is  shown  at  the  top  of  the  diagram.  Note 
that  the  temperature  was  practically  the  same  on  both  nights,  a  few 
minutes  after  7  P.M. 

Heat  is  also  released  during  the  process  of  freezing  of  water,  in 
the  same  way  that  it  is  released  when  dew  or  white  frost  is  formed. 
The  amount  of  heat  liberated  in  the  process  of  freezing  a  given 
amount  of  water  is  slightly  more  than  one-seventh  as  much  as  that 
liberated  in  the  condensing  process.  However,  when  the  ground  over 
a  large  area  is  wet  from  previous  heavy  rains,  the  amount  of  heat 
given  off  in  the  freezing  of  the  surface  moisture  is  sometimes  sufficient 
to  hold  the  temperature  stationary  near  the  freezing  point  for  two 
or  three  hours.  Evaporation  from  a  wet  surface  soil  interferes  with 
the  normal  rise  in  temperature  during  the  day,  particularly  if  the 
air  is  relatively  dry  and  a  strong  wind  is  blowing.  However,  a 
thoroughly  saturated  surface  soil  is  far  more  effective  in  preventing 
the  occurrence  of  an  extremely  low  temperature  at  night  through  the 
release  of  heat  in  the  freezing  process,  than  it  is  in  cooling  the  air  dur- 
ing the  day  through  evaporation.  The  greatest  danger  of  extremely 
low  temperatures  in  California  comes  when  both  the  soil  and  the  air 
are  very  dry. 

A  heavy  blanket  of  low  clouds  composed  of  particles  of  liquid 
moisture  practically  eliminates  the  radiation  of  heat  from  the  ground 
for  which  reason  there  is  little  danger  of  frost  in  California  so  long 
as  clouds  of  this  type  exist.  Thin,  high  clouds  often  are  composed  of 
tiny  ice  particles,  which  do  not  interfere  materially  with  the  escape 
of  the  heat  from  the  earth  by  radiation,  and  heavy  frosts  often 
occur  when  the  sky  is  entirely  overcast  with  clouds  of  this  type. 

The  occurrence  of  a  heavy  frost  accompanied  by  a  strong  wind 
is  extremely  rare  in  the  fruit-growing  sections  of  California.  Frosts 
usually  occur  on  calm  nights,  and  even  light  gusts  of  wind  usually 
cause  the  temperature  to  rise  rapidly  above  the  danger  point.  This 
effect  is  due  to  the  mixing  of  the  warmer  air  above  the  orchards  with 
the  cold  air  down  among  the  trees.  The  "freezes,"  accompanied  by 
strong  winds,  which  may  occur  at  intervals  of  from  fifteen  to  twenty- 
five  years,  make  necessary  the  most  strenuous  efforts  to  protect  fruit 
and  trees  by  orchard  heating. 

When  to  Expect  Frost  in  California.  The  weather  in  California 
is  controlled  by  atmospheric  disturbances  of  wide  extent  and  varying 


BULL.  398]  ORCHARD   HEATING    IN    CALIFORNIA  21 

intensity,  which  move  down  the  coast  from  the  vicinity  of  Alaska, 
or  inland  from  the  Pacific  Ocean.  These  disturbances  are  of  two 
types,  one  of  which  is  marked  by  low  barometer,  overcast  skies,  and 
rain  or  snow;  the  other  by  high  barometer  and  clear  skies. 

The  important  requirements  for  the  occurrence  of  frost,  a  clear 
sky  and  little  wind,  are  present  during  the  passage  of  an  area  of  high 
barometer.  As  the  first-mentioned  type  of  disturbance,  the  area  of 
low  barometer  with  overcast  skies  and  rain,  nearly  always  precedes 
the  area  of  high  barometer,  the  local  belief  that  frosts  are  likely  to 
follow  a  rain  has  some  basis.  In  many  cases,  however,  the  rain  area 
does  not  reach  southern  California,  and  extremely  severe  frosts  may 
occur  in  that  section  without  any  rain  preceding  them. 

During  the  passage  of  a  well-defined  area  of  low  barometer  the 
radiation  from  the  sun  is  more  or  less  completely  cut  off  by  heavy 
clouds  and  the  ground  is  not  warmed  much  during  the  day.  If  rain 
has  fallen,  the  evaporation  from  the  wet  ground  uses  up  a  great  deal 
of  heat,  and  this  also  tends  to  keep  the  temperature  low  during  the 
day.  Therefore,  during  the  frost  season,  on  the  first  clear  night  after 
a  rain  the  temperature  at  sunset  is  likely  to  be  within  15°  F.  or  20°  F. 
of  the  freezing  point  and  not  much  cooling  by  radiation  is  necessary 
to  form  frost.  Although  the  moisture  in  the  ground  after  a  rain 
tends  to  prevent  warming  of  the  ground  during  the  day,  it  also  tends 
to  prevent  a  large  fall  in  temperature  during  the  night.  The  water 
vapor  taken  up  by  the  atmosphere  from  the  wet  ground  diminishes 
radiation.  When  the  dew  point,  which  is  likely  to  be  high  under 
these  conditions,  is  reached,  the  latent  heat  released  retards  the  rate 
of  cooling  still  more,  and  when  the  freezing  point  is  reached  the 
conversion  of  the  soil  moisture  into  ice  also  liberates  heat  and  aids  in 
preventing  a  further  fall  in  temperature.  By  the  second  night  follow- 
ing the  rain  the  surface  of  the  ground  has  usually  dried  out  consider- 
ably. The  dew  point  is  likely  to  be  lower  and  a  more  damaging  frost 
is  likely  to  occur.  In  California,  before  the  third  night  the  day 
temperature  usually  has  risen  high  enough  to  make  unlikely  the 
occurrence  of  a  severe  frost,  although  there  are  exceptions  to  this 
rule.  When  the  air  and  soil  have  been  unusually  dry  and  an  area 
of  high  barometer  lies  to  the  north  or  northeast,  damaging  frosts  have 
been  known  to  occur  on  as  many  as  fifteen  successive  nights  in  the 
colder  portions  of  a  very  limited  area  in  southern  California. 

Freezes  in  California,  as  explained  previously,  are  caused  by  the 
influx  of  great  masses  of  cold  dry  air  from  the  north  and  northeast, 
together  with  local  conditions  favorable  for  the  rapid  loss  of  heat 
by  radiation.     The  progress  of  a  freeze  usually  can  be  traced  south- 


22  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

ward  from  the  Canadian  border,  and  sometimes  from  Alaska,  the 
journey  to  southern  California  often  requiring  several  days.  The 
weather  map  is  absolutely  necessary  for  forecasting  the  approach  of 
a  freeze. 

Frost  Forecasts  by  the  United  States  Weather  Bureau.  General 
weather  forecasts,  including  frost  forecasts  when  necessary,  are  issued 
twice  daily,  morning  and  evening,  from  the  regular  stations  of  the 
Weather  Bureau.  Forecasts  for  the  entire  state  are  issued  from  the 
San  Francisco  office,  while  the  forecasts  given  at  other  stations  apply 
only  to  the  immediate  vicinity  of  the  station  from  which  they  come. 
If  frost  forecasts  are  desired  for  use  in  connection  with  the  protection 
of  crops,  the  matter  should  be  taken  up  with  the  San  Francisco  office 
of  the  Weather  Bureau. 

Fruit  Frost  Service.  During  the  past  ten  years  the  Weather 
Bureau  has  maintained  a  small  corps  of  specially  trained  meteor- 
ologists in  some  of  the  fruit  growing  sections  where  interest  in  frost 
protection  has  been  sufficient  to  warrant  the  expense.  It  has  been 
the  effort  of  these  trained  men  to  remove  as'  much  as  possible  of  the 
uncertainty  connected  with  orchard  heating  operations,  and  to 
increase  their  efficiency.  The  work  has  been  conducted  along  the 
following  general  lines : 

Predicting  the  lowest  temperature  to  be  expected  each  night  in 

the  district  where  operations  are  conducted. 
Giving    expert    advice   to    fruit    growers    in    connection    with 

orchard  heating. 
Conducting  a  temperature  survey  of  each  district  to  determine 
temperature  differences,  and  to  enable  crop  losses  to  be  deter- 
mined accurately  immediately  after  the  frost. 
Conducting  experimental  work  in  connection  with  frost  and 
fruit  protection. 
The  experimental  work  includes  studies  on  the  subjects  mentioned 
below  together  with  others  of  less  importance. 

Increasing  the  accuracy  of  the  minimum  temperature  fore- 
casts for  each  district. 
Determining    more    accurately    the    temperatures    at    which 

damage  occurs  to  buds,  blossoms,  and  fruit. 
Testing  new  devices  for  frost  protection. 
Developing   improved    types    of    thermometers    for    orchard 

heating  work. 
Determining  the  influence  of  covercrops  on  the  frost  hazard. 


Bull.  398]  ORCHARD   HEATING   IN    CALIFORNIA  23 

Measuring  the  amount  of  temperature  inversion  on  different 

nights,  and  determining  the  causes  for  variations. 
Determining  the  character  of  air  drainage  on  different  types 

of  slopes. 
Measuring  the  value  of  the  smoke  cover  in  preventing  radia- 
tion of  heat  from  the  ground  at  night. 
The  fruit  frost  work  has  in  all  cases  been  carried  on  in  cooperation 
with    various    fruit    growers'    organizations    or    other    cooperative 
agencies.     The  local  financial  cooperation  amounts  to  about  one-half 
the  total  cost  of  the  service.     All  thermometers  brought  in  by  the 
fruit  growers  in  each  of  the  fruit  frost  districts  are  tested  for  accuracy 
at  the  beginning  of  the  frost  season  by  the  fruit  frost  specialist,  with- 
out charge.     The  tests  have  shown  that  a  considerable  percentage  of 
the  thermometers  used  in  orchard  heating  are  inaccurate,  although 
there  has  been  a  remarkable  improvement  in  this  respect  during  the 
last  few  years. 


METHODS  OF  PREVENTING  HEAT  LOSSES  AND  OF  ADDING  HEAT 

From  the  foregoing  discussion  it  will  be  seen  that  frost  occurs 
mainly  because  of  heat  losses  through  radiation  and  that  protection 
can  be  obtained  only  by  prevention  of  these  losses  or  by  the  addition 
of  sufficient  heat  to  make  up  for  losses  which  would  otherwise  cause 
the  temperature  to  fall  below  the  danger  point. 

Prevention  of  Heat  Losses.  The  greenhouse  is  an  application  of 
the  principle  of  reducing  radiation  losses.  Glass  is  transparent  to 
the  heat  waves  of  short  wave  length  radiated  by  the  sun,  but  is  prac- 
tically opaque  to  the  longer  waves  re-radiated  from  the  soil.  There- 
fore a  glass  screen  is  very  effective  in  preventing  loss  of  heat,  but  is 
not  practicable  for  an  orchard.  Different  kinds  of  cloth  and  lath 
screens  have  been  tried  for  the  same  purpose  with  but  little  effect 
on  account  of  the  fact  that  radiation  takes  place  from  the  outside 
of  the  screens  and  unlike  glass,  they  are  unable  to  prevent  an  inter- 
change of  air  between  the  protected  area  and  the  outside.  Heavy 
cloth  screens  so  near  the  ground  that  there  is  little  movement  of  air 
beneath  afford  considerable  protection  to  tender  vegetables. 

Water  vapor  is  very  effective  in  retarding  loss  of  heat  by  radia- 
tion; for  this  reason  various  means  of  increasing  the  humidity  of 
the  air  have  been  tried.  When  serious  frosts  occur  in  California, 
the  air  is  generally  very  dry  and  any  moisture  added  is  rapidly  lost 
by  diffusion  and  air  movement  so  that  it  is  seldom,  if  ever,  possible 


24  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

to  produce  fog  artificially  on  frost  nights.  It  was  formerly  thought 
that  the  smoke  from  smudge  pots  was  effective  in  preventing  loss  of 
heat  by  radiation  but  careful  measurements  by  the  Weather  Bureau 
have  shown  that  the  densest  smoke  screens  produced  by  smudge  pots 
are  effective  only  in  decreasing  the  rate  of  radiation  about  10  per 
cent,  and  that  the  final  temperature  reached  under  a  smoke  screen 
may  be  as  low  as  under  a  clear  sky.  The  only  real  benefit  to  be  derived 
from  cloth  or  lath  screens  and  to  a  lesser  degree  from  smoke  screens 
is  a  slowing  down  of  the  thawing  process  in  frozen  fruits  and  blossoms 
with  a  consequent  lessening  of  the  damage.  A  lath  covering  makes 
heating  easier  and  lath-protected  nursery  stock  is  easily  saved  by 
heaters. 

The  principle  of  prevention  of  heat  losses  is  applied  to  the 
protection  of  young  trees  by  means  of  wrapping  the  trunks  with 
insulating  materials.  Some  growers  use  thick  bundles  of  newspaper, 
but  corn  stalks,  Sudan  grass,  or  tules  are  preferable.  The  wrappings 
should  be  tight  enough  to  prevent  air  movements.  In  applying  corn 
stalks  or  similar  materials  enough  should  be  placed  around  the  trunk 
of  one,  two,  or  three-year-old  trees  to  make  a  covering  about  three 
inches  thick  on  all  sides.  They  should  be  tied  firmly  at  the  bottom, 
middle,  and  top  of  the  trunk,  the  tops  of  the  stalks  extending  up 
through  the  branches.  Treatment  of  this  kind  will  prevent  trunk 
damage  and  reduce  injury  to  the  branches.  If  the  latter  are  frozen 
back  a  new  head  can  be  grown  on  the  sound  trunk  and  the  tree  saved. 
Young  citrus  trees  should  be  protected  in  this  manner  in  all  districts 
in  California.  In  windy  districts  it  is  advisable  to  tie  the  wrapped 
trees  to  stout  stakes  such  as  grape  stakes,  because  the  wrappings 
greatly  increase  the  danger  of  blowing  over.  The  wrappings  should 
be  applied  in  November  and  removed  as  soon  as  all  danger  of  severe 
frost  is  past.  If  left  on  too  late  in  the  spring,  during  periods  of  rainy 
weather,  they  may  promote  infection  with  various  bark  diseases. 

Addition  of  Heat.  The  addition  of  heat  to  the  air  in  the  area  to 
be  protected  is  the  only  practical  means  so  far  developed  for  protect- 
ing a  large  area.  As  pointed  out  previously  it  is  not  necessary  to 
"heat  all  out-of-doors"  in  order  to  raise  temperatures  above  the 
danger  point.  Many  methods  of  adding  heat  to  the  orchard  air 
have  been  suggested,  the  more  important  of  which  will  be  discussed 
in  the  following  paragraphs.  Almost  every  year  new  devices  are 
suggested,  some  for  adding  heat,  and  some,  according  to  the  inventors, 
designed  to  absorb  cold.  Since  cold  is  merely  the  absence  of  heat, 
obviously  it  cannot  be  absorbed  and  therefore  devices  which  do  not 
add  heat  can  hardly  be  expected  to  succeed. 


Bull.  398] 


ORCHARD    HEATING    IN    CALIFORNIA 


25 


Use  of  Water.  The  spraying  of  trees  with  water  has  been  tried 
as  a  means  of  adding  heat  to  the  air.  The  water  generally  is  at  a 
temperature  considerably  above  the  freezing  point  and  gives  up  heat 
as  it  cools,  and  in  addition  gives  up  its  latent  heat  when  it  freezes, 
thus  greatly  delaying  the  fall  of  temperature  below-  32°  P.  Latent 
heat  is  that  heat  which  is  associated  with  a  change  of  physical  state. 
A  pound  of  water  changing  to  ice  liberates  144  times  as  much  heat 
as  it  does  in  falling  one  degree  Fahrenheit.  Starting  with  water  at 
62°  F.,  cooling  it  to  the  freezing  point  and  then  freezing  all  of  it 


Fig.  3. — Tree  broken  down  by  ice  formed  by  sprinkling  with  water  in  an 
attempt  to  protect  it  from  frost. 

liberates  about  10,800  B.T.IL  per  cubic  foot.  The  cooling  and  freezing 
of  twelve  cubic  feet  of  water  will  just  about  equal  the  burning  of 
one  gallon  of  oil.  This  cannot  be  looked  upon  as  a  net  gain  of  heat 
since  part  of  the  water  may  evaporate,  giving  a  cooling  effect.  The 
evaporation  of  one  cubic  foot  of  water  absorbs  heat  equivalent  to  the 
burning  of  one-half  gallon  of  oil.  The  melting  of  a  pound  of  ice 
absorbs  144  B.  T.  IT.  Therefore,  if  much  ice  is  formed  when  water 
is  used  there  is  little  gain  of  heat  from  the  sun  the  next  day  and  the 
soil  is  kept  cool,  so  that  more  water  or  more  fuel  will  be  required 
if  freezing  temperatures  occur  the  next  night.  Unfortunately  even 
where  water  is  available  for  sprinkling,  the  weight  of  ice  formed  is 
so  great  as  to  severely  damage  or  even  totally  destroy  trees.  Figure 
3  shows  a  tree  thus  destroyed. 


26  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Water  in  basins  or  running'  in  furrows  liberates  a  like  quantity 
of  heat  in  freezing  and  affords  a  small  amount  of  protection.  If  a 
grower  has  plenty  of  water  available  and  no  other  means  of  protec- 
tion he  ought  certainly  to  run  the  water  both  day  and  night  during 
the  entire  period  of  the  freeze,  but  running  water  is  to  be  looked  upon 
as  a  means  of  partial  rather  than  complete  protection. 

Blowers.  A  large  number  of  devices  have  been  tried  for  adding 
heat  to  the  air  in  the  orchard.  Among  these  are  the  blowers  or  so- 
called  "wind-jammers."  As  pointed  out  previously,  on  a  night  of  a 
radiation  frost  there  is  a  marked  temperature  inversion,  wherein 
it  is  frequently  as  much  as  ten  degrees  warmer  at  the  40-foot  level  than 
it  is  near  the  ground.  The  blowers  were  originally  designed  to  mix 
this  warmer  upper  air  with  the  cold  air  in  the  orchard,  but  they  have 
not  succeeded  in  raising  the  temperature  in  the  orchard  as  much  as  one 
degree  as  a  result  of  the  mixing.  Later  models  of  these  blowers 
include  furnaces  in  which  heat  is  produced,  but  they  have  shown 
little  if  any  improvements  in  results  over  those  merely  mixing  the  air. 

There  is  little  hope  that  any  of  these  devices  will  ever  prove 
successful  for  several  reasons.  First,  it  is  practically  impossible  to 
drive  a  current  of  warm  light  air  down  into  cold  heavy  air,  and  this 
is  especially  true  where  the  trees  are  large.  If  sufficient  power  is 
used  leaves  and  fruit  are  stripped  from  the  trees.  Second,  the 
machines  are  generally  built  to  serve  ten  acres  and  it  is  not  feasible 
to  produce  enough  heat  in  one  furnace  to  compensate  for  radiation 
losses  from  ten  acres  of  ground.  It  may  be  necessary  during  severe 
cold  spells  to  burn  oil  at  the  rate  of  500  gallons  per  hour  for  ten 
acres.  The  heat  incident  to  such  a  combustion  would  probably  melt 
the  burners  and  equipment.  The  burning  of  insufficient  fuel  would 
mean  only  partial  protection  even  if  the  heat  could  be  distributed 
throughout  the  area  to  be  protected.  Third,  the  generation  of  the 
large  quantities  of  heat  required  for  ten  or  more  acres  in  one  furnace 
causes  much  greater  losses  of  heat  to  the  upper  atmosphere  than  if 
heat  is  produced  from  a  large  number  of  small  units,  which  would 
make  the  fuel  costs  excessive.  This  difficulty  makes  any  sort  of 
central  heating  plant  for  a  large  acreage  impracticable.  An  additional 
objection  to  any  sort  of  central  device  protecting  a  considerable 
acreage  with  one  or  at  most  a  few  units  is  the  danger  of  complete 
loss  of  crop  through  the  failure  to  function  of  one  single  plant.  The 
breakage  of  even  a  minor  part  would  probably  prevent  operation  long 
enough  to  cause  severe  damage. 

Use  of  Orchard  Heaters.  The  only  method  of  getting  complete 
protection  which  has  been  successfully  demonstrated  by  field  experi- 


Bull.  398 j  ORCHARD  HEATING  IN   CALIFORNIA  27 

ence  is  to  have  a  large  number  of  small  heat  units  distributed  over 
the  area  to  be  protected.  As  explained  previously,  on  frosty  nights, 
because  of  temperature  inversion,  there  is  a  relatively  thin  layer  of 
air  below  the  danger  point  while  the  air  higher  up  may  be  at  a 
temperature  above  the  danger  point.  If  the  air  overhead  is  as  cold 
as  that  near  the  ground  successful  heating  will  be  very  difficult,  if 
not  impossible.  This  usual  layer  of  warmer  air  overhead  serves  as 
a  ceiling,  so  to  speak,  which  retains  the  heat  added  to  the  layer  below. 
By  means  of  the  heaters  all  of  the  air  under  the  il ceiling"  is  grad- 
ually warmed  until  it  is  above  the  danger  point.  Enough  fires  are 
then  maintained  to  compensate  for  losses  of  heat  through  radiation 
and  air  drift.  Success  in  orchard  heating,  therefore,  is  attained  by 
heating  a  large  volume  of  air  a  few  degrees  rather  than  by  heating 
a  small  volume  to  high  temperatures. 

There  are,  on  this  basis,  two  fundamental  requirements  for 
adequate  frost  protection:  (1)  a  sufficient  number  of  heaters  per 
acre  to  heat  a  large  volume  of  air  without  overheating  any  part,  and 
(2)  sufficient  fuel  to  keep  these  heaters  burning  the  entire  duration 
of  the  frost.  The  number  of  heaters  and  proper  field  supply  of  fuel 
will  be  discussed  on  the  basis  of  full  protection  for  cold  locations. 
Anything  short  of  full  protection  is  a  poor  investment.  One  may 
lose  an  entire  crop  as  well  as  the  money  spent  on  heating  if  the 
number  of  heaters  is  too  small  or  if  the  fuel  burns  out  before  morning. 
If  protection  is  inadequate  a  portion  of  the  crop  may  be  frozen  and 
the  grade  of  the  whole  crop  reduced.  Frequently  the  difference  in 
price  between  first  grade  fruit  and  so-called  "merchantable  grade" 
is  sufficient  to  pay  the  entire  cost  of  equipping  an  orchard  for  full 
protection. 

The  number  of  fires  per  acre  which  must  be  kept  burning  to 
heat  a  large  volume  of  air  a  few  degrees  will  vary  from  less  than 
20  to  about  100  according  to  the  size  of  the  fires,  the  degree  of  cold, 
the  crop  to  be  protected,  and  the  atmospheric  conditions  determining 
the  efficiency  of  heating,  such  as  air  drift,  humidity,  and  amount  of 
inversion.  With  a  relatively  small  number  of  fires  burning,  the 
temperature  can  be  raised  by  increasing  the  rate  of  fuel  consumption 
up  to  a  certain  point,  but  beyond  that,  further  increase  merely  results 
in  overheating  part  of  the  air  and  losing  heat  to  the  upper  atmosphere. 
Under  these  circumstances  more  heaters  should  be  lighted.  To  sum 
up,  the  basic  principle  of  frost  protection  is  to  keep  a  relatively 
large  number  of  small  heat  units  burning  the  entire  period  during 
which  the  temperature  outside  the  heated  area  is  below  the  danger 
point. 


28  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


EQUIPMENT   AND    FUELS 

The  necessary  items  of  equipment  for  orchard  heating  are  heaters, 
fuel,  storage  facilities,  filling  equipment,  torches,  torch  fuel,  and 
thermometers.  In  addition  to  these  some  growers  have  thermographs 
and  frost  alarms. 

Heaters 

Many  types  of  heaters  have  been  put  on  the  market  during  the 
last  fifteen  years.  New  types  are  brought  out  each  year,  but  we  are 
still  far  from  an  ideal  heater. 

We  have  come  to  have  rather  definite  ideas,  however,  as  to  the 
requirements  of  a  heater  for  a  citrus  orchard  under  California  con- 
ditions.   These  are  that  it  should : 

Hold   sufficient   fuel  to   burn   all   night   without   refueling,   even 
though  a  gallon  or  more  of  oil  per  hour  be  burned  at  times. 
Be  capable  of  sufficient  regulation  to  give  its  greatest  heat  just 
before  sunrise  even  though  the  fuel  in  the  reservoir  is  low 
by  this  time. 
Be  able  to  burn  any  of  the  ordinary  grades  of  heating  fuels  on  the 
market  without  leaving  a  heavy  residue  or  making  too  much 
smoke. 
Give    reasonably  complete  combustion  without  delivering  the  heat 
and  products  of  combustion  too  far  from  the  ground,  or 
heating  metallic  surfaces  so  hot  as  to  give  a  large  proportion 
of  the  total  heat  in  radiant  form. 
Be  easy  to  light  and  regulate  by  inexperienced  labor  under  all 

weather  conditions. 
Be  easy  to  take  apart,  clean,  and  store. 
Be  so  designed  that  if  it  burns  dry  the  bottom  of  the  heater  will 

not  be  damaged. 
Be  made  of  good  material  and  show  small  annual  depreciation. 
Be  of  reasonable  cost. 

No  heater  meets  all  of  these  qualifications,  and  the  grower  must 
therefore  choose  the  one  which  will  most  nearly  meet  his  conditions. 

At  the  present  time  there  are  a  number  of  types  to  choose  from, 
the  principal  features  and  adaptations  of  which  are  as  follows : 

The  Lard  Pail  Heater.  "Bolton"  and  "Canco"  types  (figure 
4a).  This  heater  is  a  simple  pail  with  sloping  sides,  a  spider  or  flame 
spreader  and  a  cover.  It  is  made  in  two  sizes,  holding  five  and  ten 
quarts  of  oil.    A  few  eight-quart  heaters  of  this  type  are  in  use.    This 


BULL.  398]  ORCHARD   HEATING   IN    CALIFORNIA  29 

type  is  very  efficient  in  "heating  a  lot  of  air  a  little,"  and  burns 
satisfactorily  the  present  type  of  fuel  oils  of  27°  Baume  or  higher. 
It  apparently  gives  as  much  heat  per  gallon  as  the  newer  and  larger 
types  of  heaters.  It  meets  the  last  six  of  the  nine  requirements  listed 
above.  The  chief  objections  to  it  are  that  it  will  not  burn  all  night 
without  refilling  and  that  it  produces  a  smudge.  It  is,  however, 
very  satisfactory  for  deciduous  fruit  orchards,  because  it  is  economi- 
cal, and  quite  as  efficient  as  more  expensive  types.  Deciduous  fruit 
orchards  do  not  usually  require  long  hours  of  burning  and  since  there 
is  no  ripe  fruit  on  the  trees  when  frosts  occur,  the  smudge  does  not 
lower  the  grade  of  the  fruit.  For  deciduous  fruit  orchards  80  to  100 
of  the  ten-quart  size  should  be  used  per  acre  or  150  of  the  five-quart 
size.  These  sizes  actually  hold  eight  and  four  quarts  of  oil  respec- 
tively when  filled  to  proper  level.  The  time  of  burning  is  about 
3  to  31, 2  hours  for  both  sizes  without  the  spider  and  9  to  10  hours  with 
the  spider.  The  spider  is  put  on  to  reduce  the  rate  of  burning,  but 
is  rarely  used  except  with  very  light  oil.  Obviously  the  ten-quart 
heater  produces  heat  twice  as  rapidly  as  the  five-quart  size.  In  order 
to  prolong  the  hours  of  burning,  or  provide  greater  heat  just  before 
sunrise,  a  portion  of  the  heaters  are  held  in  reserve  so  that  additional 
heaters  can  be  lighted  after  those  first  lighted  have  burned  low. 

Low  Stack  Distilling  Type  Oil  Heaters  of  from  Seven  to  Ten 
Gallons  Capacity  (figure  4 — 0,  c,  d).  This  and  the  two  following 
types  burn  in  the  reservoir  with  just  enough  fire  to  generate  gases 
which  burn  in  or  above  the  stack.  Two  makes  of  this  type  are  now 
on  the  market,  and  there  are  several  other  makes  previously  sold  and 
still  in  general  use.  This  type  of  heater  is  very  satisfactory.  The 
chief  objection  to  the  low  stack  heater  is  that  most  representatives  of 
this  type  produce  considerable  smoke,  which  is  objectionable  in 
thickly  settled  regions,  and  also  when  the  heaters  are  burned  for 
several  nights  the  fruit  becomes  coated  with  soot,  which  increases  the 
difficulty  of  washing.  Some  makes  also  have  the  drawback  of  choking 
the  stacks  with  soot  which  often  makes  it  necessary  to  clean  them 
while  still  burning.  On  the  whole,  howTever,  heaters  of  this  type 
have  proved  efficient  and  easy  to  handle  and  are  very  popular  with 
the  growers.  They  are  the  lowest  in  first  cost  of  any  of  the  large 
capacity  heaters  and  because  no  parts  get  excessively  hot  the  deprecia- 
tion rate  is  low.  It  is  possible  to  convert  some  makes  of  this  type  to 
the  medium  stack  type.  Some  growers  who  have  bought  low  stack 
heaters  in  order  to  get  protection  at  low  cost  convert  them  later  to 
reduce  the  smoke  nuisance. 


30  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Medium  Stack  Down  Draft  Distilling  Oil  Heaters  of  from  Seven 
to  Ten  Gallons  Capacity  (figure  4e).  The  so-called  "Supply  Com- 
pany" heater  is  an  example  of  this  type.  Certain  earlier  models 
were  difficult  to  light  and  regulate  and  also  sooted  up  badly  but 
the  newer  models  burn  freely,  make  only  a  small  amount  of  smoke 
if  properly  regulated,  and  are  generally  efficient.  They  meet  most 
of  the  requirements  listed  above.  The  disadvantages  over  the  short 
stack  type  are  that  the  hot  products  of  combustion  are  delivered  at  a 
greater  height  from  the  ground  and  there  is  somewhat  more  radiant 
heat,  a  portion  of  which  is  lost  directly  to  the  sky  without  heating 
the  air.  The  cost  is  higher  and  the  annual  rate  of  depreciation  heavier 
than  with  the  low  stack  types.  They  are  popular,  however,  because 
they  are  efficient  and  are  reasonably  smokeless. 

Tail  Stack  Distilling  Oil  Heaters.  These  heaters,  with  a  three- 
joint  stack,  are  no  longer  manufactured  and  are  rather  unpopular 
because  of  the  strong  upward  draft  which  dissipates  much  of  the  heat 
into  the  upper  atmosphere.  Where  these  heaters  are  still  used  many 
of  the  growers  pull  off  the  top  joint  of  pipe,  thus  making  medium 
stack  heaters.  If  this  is  done  special  precautions  must  be  taken  to 
prevent  sooting  up  at  the  stack  collar.  These  heaters  require  frequent 
regulation. 

Large  Capacity  Heaters  with  Burner  and  Oil  Reservoir  Separate. 
There  are  at  present  three  types  of  these,  one  feeding  oil  through 
a  needle  or  shut-off  valve,  one  a  vacuum  feed  (figure  4/)  and  one 
a  gravity  feed  (figure  4g).  The  main  advantage  of  this  general 
type  is  that  clean  oil  of  constant  composition  is  burned,  while  in  the 
other  types  the  lighter  fractions  of  the  oil  are  distilled  off  first. 
Another  advantage  is  low  depreciation  on  the  oil  reservoir.  Some 
makes  are  smokeless  and  others  very  smoky,  according  to  the  efficiency 
of  the  burner.  There  are  certain  disadvantages  to  be  found  in  heaters 
of  this  type.  The  makes  with  valves  for  feeding  oil  require  constant 
regulation  as  the  viscosity  and,  therefore,  the  rate  of  flow  of  the  oil 
changes  as  the  oil  warms  up.  The  vacuum  feed  type  is  cumbersome 
and  hard  to  fill  but  burns  all  right  if  the  oil  is  free  from  water  and 
asphaltum.  If  water  is  present  it  may  freeze  and  prevent  discharge 
of  oil;  asphaltum  may  gradually  work  back  into  and  clog  the  feed 
pipe  with  some  types  of  burners.  The  gravity  feed  type  is  subject  to 
the  very  serious  disadvantage  of  delivering  fuel  to  the  burner  less 
than  half  as  rapidly  when  the  reservoir  approaches  emptiness  as  when 
it  is  full.  In  the  morning  when  the  need  for  heat  is  greatest  the  oil 
flows  at  the  lowest  rate.  This  makes  it  imperative  to  fill  the  heaters 
as  full  as  possible  every  day. 


Bull.  398] 


ORCHARD    HEATING    IN    CALIFORNIA 


31 


Fig.  4. — Types  of  orchard  heaters  in  common  use.  a.  Lard  pail  heaters, 
" Bolton"  and  "Canco"  types,  b.  Dunn  heater  with  umbrella  cover  removed, 
c.  ' '  Citrus ' '  heater,  showing  low  stack  and  medium  stack  attachment.  D.  Scheu 
"Double  Stack"  heater  with  outer  stack  removed.  E.  Scheu  "Baby  Cone"  or 
latest  "Supply  Company"  heater  model,  f.  Scheu  vacuum  feed.  G.  Kittle 
heater.     H.  Briquet  heater. 


32  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Briquet  and  Coal  Burners.  Some  of  the  first  orchard  heating 
was  done  with  coal  baskets  but  these  were  abandoned  because  they 
were  hard  to  handle  and  of  too  small  capacity.  Coal  heaters  with 
grates  were  used  for  years  but  they  have  now  been  discarded,  largely 
because  of  the  extremely  high  labor  charges  and  the  difficulties  in 
lighting  and  extinguishing  them.  Another  serious  disadvantage  is 
the  variation  in  the  rate  of  burning.  Some  heaters. burn  rapidly 
while  others  smoulder  for  hours.  This  variation  is  due  to  variation  in 
the  draft  produced  by  uneven  sizes  of  pieces  of  coal.  With  the 
development  of  uniform-sized  briquets  of  both  carbon  black  and  coal, 
there  has  been  renewed  interest  in  solid  fuel.  Several  types  of 
heaters  burning  either  coal  or  carbon  briquets  are  now  on  the  market 
(figure  4/i).  They  are  effective  if  used  at  the  rate  of  twice  as  many 
per  acre  as  oil  heaters,  and  have  the  advantage  of  using  a  fuel  which 
is  easily  stored  and  distributed  in  the  field.  The  first  cost  is  less  than 
that  of  large  capacity  oil  heaters  even  though  twice  as  many  heaters 
are  required.  They  also  make  much  less  soot  than,  the  oil  burning 
heaters  and  have  the  added  advantage  of  delivering  the  heat  near 
the  ground.  The  principal  disadvantages  of  this  type  of  heater  as 
compared  with  oil  burning  types  are  that  more  labor  is  required  for 
lighting  and  handling;  frequent  refueling  is  necessary  to  maintain 
constant  heat,  particularly  if  coal  briquets  are  used;  the  fuel  costs 
are  higher  in  California  for  the  same  amount  of  heat ;  and  fuel  losses 
are  greater  because  the  heaters  must  be  kept  well  filled  until  after 
sunrise,  and  the  fires  are  difficult  to  extinguish. 

Heaters  of  this  type  are  particularly  adaptable  to  small  orchards 
(three  to  five  acres)  when  the  owner  has  plenty  of  help  for  firing  and 
refueling  but  no  facilities  for  handling  oil. 

Choice  of  Heaters.  In  choosing  heaters  growers  should  endeavor 
to  meet  all  the  requirements  for  their  conditions  at  the  lowest  possible 
cost,  taking  into  consideration  first  cost,  annual  depreciation  and 
operating  costs.  For  oranges  40  to  50  large  capacity  oil  burners 
per  acre  will  be  needed  (more  for  very  cold  locations),  and  for  lemons 
and  avocados  60  to  80.  If  briquet  heaters  are  used  the  number  should 
be  doubled.  For  deciduous  fruits  the  range  will  be  150  five-quart  lard 
pails,  80  to  100  ten-quart  lard  pails,  or  80  briquet  heaters.  The 
recommendations  for  deciduous  fruits  apply  to  berries  and  vegetables 
in  a  general  way.  These  recommendations  are  based  on  field  experi- 
ence as  to  number  of  heaters  required  for  cold  locations.  Local 
experience  may  indicate  that  one  can  succeed  with  a  smaller  amount 
of  equipment  than  that  suggested  here  as  necessary  for  full  protec- 
tion, but  it  must  be  remembered  that  different  districts  have  been 


Bull.  398]  ORCHARD   HEATING   IN    CALIFORNIA  33 

the  cold  areas  in  different  years  and  that  a  fair  margin  of  safety 
beyond  the  usual  expectation  must  be  provided. 

Initial  cost  is  an  important  item  and  should  be  kept  down  as  far 
as  is  consistent  with  full  protection.  In  many  districts  the  number 
of  heaters  lighted  is  rarely  more  than  from  twenty  to  twenty-five 
per  acre  at  any  one  moment  but  severe  frosts  have  been  known  in 
those  same  districts  which  would  require  fifty  heaters  per  acre  for  at 
least  two  or  three  hours  and  sometimes  longer.  Some  growers  have 
taken  advantage  of  this  situation  by  installing  20  to  25  large  capacity 
modern  oil  heaters  per  acre  to  take  care  of  the  usual  burning.  They 
then  fill  in  with  smaller  heaters  or  old  types  filled  with  oil  or  briquets, 
for  emergency  use. 

Hillsides  and  high  locations  have  lower  frost  hazards  than  level 
fields  and  so-called  "frost  pockets/'  but  they  also  have  less  favorable 
temperature  inversions.  Therefore  heating  the  former  locations,  while 
not  often  necessary,  will  be  more  difficult  than  heating  the  latter. 
Even  though  the  temperature  is  not  likely  to  fall  so  low  on  the 
hillsides  as  in  the  "frost  pockets"  as  many  heaters  per  acre  are 
likely  to  be  required  when  heating  becomes  necessary. 

Deciduous  fruit  growers  are  rarely  justified  in  the  purchase  of 
more  expensive  heaters  than  the  lard  pail  types  which  are  very 
efficient  for  their  needs.  However,  where  it  is  difficult  to  haul  oil 
into  wet  orchards  or  orchards  on  rolling  ground  some  growers  use 
large-size  heaters,  holding  from  8  to  10  gallons  each,  at  the  rate  of 
about  20  heaters  per  acre,  and  then  fill  in  with  30  to  40  ten-quart 
lard  pails  for  use  in  emergencies  when  more  than  twenty  sources  of 
heat  are  required.  In  this  way  they  get  through  the  season  with  fewer 
refillings.  Because  of  the  difficulty  of  filling  oil  heaters  on  wet  heavy 
soil,  briquet  heaters  are  sometimes  used  as  emergency  reserves  in  the 
same  way. 

Fuels 

Any  sort  of  fuel  which  can  be  kept  burning  in  properly  distributed 
heaters  without  too  much  difficulty  will  raise  the  temperature,  but 
oil  is  the  most  popular  because  it  is  cheap,  easy  to  get  at  all  times, 
easy  to  light  and  extinguish,  and  easy  to  handle.  The  oil  used  for 
orchard  heating  should  be  a  wholly  distilled  product  sold  under  the 
name  of  orchard  heating  oil  or  Diesel  oil,  varying  from  24°  to  36°, 
preferably  above  28°  Baume.  It  should  be  practically  free  from  water 
and  asphaltum.  The  better  the  grade  the  less  trouble  will  result  from 
smoke  and  non-burnable  residue. 


34  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT    STATION 

Coal  and  carbon  briquets  are  obtainable  in  southern  California 
at  about  the  same  prices.  If  carbon  briquets  are  used  they  should 
be  bought  well  in  advance  of  the  heating  season  so  as  to  be  thoroughly 
cured  and  free  from  water. 

Electricity  has  been  suggested  many  times  as  a  source  of  heat. 
Electrical  heaters  were  tried  in  a  small  way  at  Riverside  in  1913. 
The  use  of  one-horsepower  per  tree  prevented  tree  damage  but  did 
not  save  any  fruit.  The  electrical  equivalent  of  20  gallons  of  oil  per 
acre  per  hour  burned  at  full  efficiency  is  a  continuous  current  flow 
of  about  750  kilowatts  or  1000  horsepower  per  acre. 

Comparative  heat  values  for  different  fuels,  assuming  complete 
combustion,  are  as  follows : 

1  gallon  average  orchard  heating  oil  equals 

9  to  10  lbs.  carbon  briquets 

10  toll  lbs.  coal  briquets 
16  lbs.  dry  oak  wood 

14  lbs.  dry  pine  wood 
37  kilowatt  hours  electric  energy. 
Fuel  Requirements  Per  Acre.  In  addition  to  having  enough  fires 
per  acre  there  must  be  enough  fuel  actually  on  hand  in  the  field  to 
burn  the  entire  duration  of  the  cold  on  any  one  night  if  full  protec- 
tion is  to  be  had.  With  the  different  fruits  in  cold  districts  for  full 
protection  the  following  amounts  of  fuel  per  acre  might  be  required 
during  one  severe  night ;  they  therefore  constitute  safe  limits  of  field 
fuel  capacity  or  fuel  actually  in  the  field  and  ready  for  use  every 
night. 

Oranges.    Oil :  400  to  450  gals,  in  heaters.    Briquets :  1500  to  2000 

lbs.  in  heaters,  1500  to  2000  lbs.  near  heaters  in  field. 
Lemons  and  Avocados.    Oil :  600  gals,  of  oil  in  heaters. 

(Briquets    probably    of     doubtful    utility    for    lemons     and 
avocados. ) 
Deciduous  Fruits.     Oil:    150  to  200  gals,  in  heaters.     Briquets: 

1250  lbs.  in  heaters,  800  lbs.  near  heaters  in  field. 
Refilling  should  take  place  after  each  burning  and  every  possible 
effort  should  be  made  to  keep  the  field  fuel  supply  at  a  safe  point. 

Storage.  The  customary  reserve  storage  on  hand  or  in  the  com- 
munity storage  reservoirs  nearby  should  be  two  or  three  times  the 
heater  capacity  for  oil,  or  the  whole  amount  of  solid  fuel  the  grower 
might  reasonably  expect  to  burn  during  the  season.  If  the  orchard 
is  far  from  the  source  of  oil  supply  even  more  than  three  times  the 
heater  capacity  should  be  carried  in  reserve.     The  usual  oil  con- 


Bull.  398] 


ORCHARD    HEATING    IN    CALIFORNIA 


35 


tainers  are  rivetted  galvanized  iron  tanks.  Since  haste  is  required 
in  refilling  heaters  after  a  long  cold  night,  it  is  advisable  to  have 
storage  tanks  elevated  so  as  to  deliver  oil  rapidly  into  the  wagon 
tanks  by  gravity.  If  elevated  storage  is  not  used,  motor-driven 
pumps  are  recommended  as  of  assistance  in  refueling  operations. 
If  the  pump  is  driven  by  a  portable  gasoline  engine  and  mounted 
on  a  suitable  carriage,  it  can  be  used  for  taking  up  oil  in  the 
spring.  Many  growers,  especially  those  on  heavy  soils,  where  hauling 
is  difficult,  have  small  storage  tanks  scattered  throughout  the  grove. 


Fig.  5. — Briquet  heater  with  funnel  to  facilitate  filling  and  tub  for  fuel  storage. 


If  these  tanks  can  be  secured  cheaply  enough  their  use  may  be 
practicable  as  a  means  of  reducing  the  amount  of  hauling  when  the 
soil  is  wet.  At  present  prices  for  new  tanks  they  cannot  be  recom- 
mended. The  importance  of  having  an  adequate  fuel  supply  near  the 
heaters  cannot  be  overestimated. 

Filling  Facilities.  Solid  fuel  heaters  are  filled  from  a  reserve 
supply  usually  stored  in  the  orchard  near  the  heaters.  The  filling 
of  heaters  during  the  day  is  facilitated  by  the  use  of  large  funnels 
fitting  into  the  heater  tops  (figure  5). 


36 


UNIVERSITY    OF    CALIFORNIA— EXPERIMENT    STATION 


The  reserve  supply  of  briquets  may  be  stored  under  the  trees  in 
sacks,  boxes  or  old  tubs.  Refueling  of  burning  heaters  is  accom- 
plished by  throwing  in  enough  briquets  to  make  a  new  layer  over  the 
fire.    A  double  layer  is  likely  to  slow  down  the  fire  too  much. 

Filling  of  oil  heaters  is  accomplished  by  the  use  of  tanks  of 
various  sorts  on  horse  or  tractor  drawn  wagons  or  light  trucks.  The 
most  popular  tanks  size  is  463  gallons,  but  the  size  varies  according 
to  soil  type  and  other  factors.     Figure  6  shows  a  tank  wagon  fitted 


Fig.  6. — Tank  wagon  used  for  filling  heaters  and  also  for  taking  up  oil. 


with  a  sludge  box,  hose  and  filling  buckets,  and  a  hand  pump  for 
taking  up  oil.  The  oil  is  carried  in  five-gallon  filling  buckets  from 
the  wagon  to  the  heater  or  is  run  through  l1/^  or  2-inch  hose.  For 
hose  filling  it  is  necessary  to  drive  along  every  row  of  heaters;  if  the 
soil  is  wet  many  growers  prefer  to  use  filling  pails,  the  oil  being 
carried  two  rows — in  rare  cases,  three  rows — on  each  side  of  the 
driveway.  The  majority  of  growers  report  that  filling  can  be  accom- 
plished much  more  quickly  with  pails,  but  the  work  is  harder.  A 
team  with  a  463-gallon  tank  and  a  crew  of  four  men  carrying  oil 
in  pails  can  distribute  about  4000  gallons  of  oil  a  day  if  the  haul  is 


Bull.  398] 


ORCHARD    HEATING    IN    CALIFORNIA 


37 


not  too  far  and  the  tank  can  be  refilled  quickly.  One  tank  and  crew 
will  take  care  of  ten  to  fifteen  acres  of  oranges.  Labor  and  teams 
for  filling  are  becoming  a  problem  in  many  districts,  and  the  grower 
should  make  definite  arrangements  for  these  services  well  in  advance 
of  the  danger  period. 


Fig.    7. — Lighting   torch. 


Torches  and  Torch  Fuels.  Lighting  is  accomplished  by  the  use 
of  torches  which  drip  burning  torch  fuel  into  the  heaters  (figure  7). 
A  torch  consists  of  a  container  with  a  spout,  a  wick,  and  a  wire  gauze 
in  the  base  of  the  spout.  The  wick  is  made  of  asbestos,  usually 
wrapped  in  a  piece  of  screen.  It  is  placed  either  directly  in  the  spout, 
loosely  enough  so  that  the  fuel  will  flow  freely  through  it,  or  in  a  slot 
close  to  the  end  of  the  spout.  In  either  case  the  wick  must  be  so 
arranged  that  the  fuel  leaving  the  spout  flows  over  or  through  it. 
The  lighted  wick  ignites  the  torch  fuel  as  it  flows  out.     The  most 


38  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

important  feature  of  the  torch  is  the  protective  wire  gauze  at  the 
base  of  the  spout  and  under  no  circumstances  should  a  grower  use 
a  torch  in  which  the  wire  gauze  is  lacking  or  defective.  The  gauze 
is  of  fine  mesh  brass  or  copper  screen  and  is  generally  soldered  into 
the  base  of  the  spout.  It  works  on  the  same  principle  as  the  miner's 
safety  lamp,  the  flame  of  the  burning  gas  being  cooled  below  the 
kindling  point  by  the  screen  and  not  passing  through  to  the  reservoir 
below.  Slight  explosions  sometimes  occur  in  the  spout  but  the  gauze 
prevents  a  disastrous  explosion  of  the  container.  The  spout  should 
screw  tightly  into  the  container  against  a  metal  gasket.  The  torches 
are  usually  filled  with  a  mixture  of  equal  parts  of  gasoline  and 
kerosene.  This  mixture  will  carry  fire  clear  to  the  ground  if  poured 
from  a  burning  torch,  and  give  a  hot  enough  fire  to  light  heaters 
readily.  An  extra  supply  of  well-mixed  torch  fuel  should  be  kept  in 
some  tight  container  such  as  a  five-gallon  can  or  a  fifteen-gallon  oil 
drum  in  the  field.  It  is  a  wise  precaution  to  fill  torches  only  by 
electric  light. 

Thermometers.  Accurate  thermometers  are  a  very  important 
part  of  orchard  heating  equipment.  After  every  freeze  it  is  possible 
to  find  examples  of  losses  of  fruit  or  needless  burning  of  oil  by 
growers  who  either  have  no  thermometers  at  all  or  depend  upon 
inaccurate  ones.  The  grower  cannot  afford  to  economize  on  ther- 
mometers. Each  ten-acre  block  should  have  three  or  four,  one  of 
which  should  be  placed  outside  of  the  heated  area  as  a  check.  If 
the  orchard  is  on  sloping  ground  a  thermometer  should  be  supplied 
for  every  change  in  elevation  of  fifty  feet. 

The  horizontal  alcohol  minimum  thermometer,  which  indicates 
the  lowest  temperature  reached,  is  the  most  satisfactory  type.  The 
minimum  temperature  is  marked  by  a  glass  indicator  which  is  set 
at  the  top  of  the  alcohol  column  by  turning  the  thermometer  upside 
down.  The  thermometer  is  then  placed  with  the  bulb  about  one  inch 
lower  than  the  top  of  the  stem  and  as  the  temperature  falls  the  glass 
indicator  is  pulled  down  by  surface  tension.  When  the  temperature 
goes  up  again  the  alcohol  flows  past  the  glass  indicator  leaving  it  in 
such  position  that  the  top  end  indicates  the  lowest  temperature 
reached  since  the  last  setting.  Note  the  indicator  in  figure  8.  Ther- 
mometers of  this  type  designed  by  the  United  States  Weather  Bureau 
especially  for  orchard  heating  work,  cost  about  $3.00  each.  One  or 
two  of  these  in  an  orchard  may  be  supplemented  by  somewhat  cheaper 
vertical  short  range  instruments,  also  of  Weather  Bureau  design. 
Even  carefully  constructed  thermometers  are  sometimes  inaccurate, 


Bull.  398] 


ORCHARD    HEATING    IN    CALIFORNIA 


39 


so  it  is  advisable  to  have  all  thermometers  tested  when  purchased, 
and  before  each  danger  season.  The  Fruit  Frost  Service  of  the 
Weather  Bureau  performs  this  service  free  of  charge  in  all  districts 
where  they  have  field  men.  If  there  is  no  local  representative  of 
the   Fruit   Frost   Service   in   the   district,   the   county    farm   advisor 


Fig.  8. — Above:  Minimum  thermometer  in  simple  shelter.     Below 
showing  position  of  thermometer. 


Detail 


sometimes  arranges  to  have  thermometers  sent  to  the  nearest  place 
where  tests  are  made. 

If  thermometers  are  so  placed  in  the  orchard  that  the  bulbs  are 
exposed  to  the  sky  they  will  lose  heat  by  radiation,  the  amount 
depending  on  the  type  of  instrument  used,  and  the  temperature 
recorded  will  therefore  be  the  radiation  temperature  of  the  ther- 
mometer.    This  may  vary  as  much  as  three  or  four  degrees  from 


40 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


the  true  air  temperature.  To  provide  accurate  reading  thermometers 
must  be  placed  in  shelters  which  shade  them  from  the  sky  and  also 
prevent  the  formation  of  dew  on  the  bulbs.  A  very  satisfactory 
shelter  may  be  made  from  two  thin  boards  about  9  or  10  inches  wide 
and  16  or  18  inches  long.  One  board  is  placed  at  hight  angles  to 
the  other,  one  constituting  the  back  of  the  shelter,  the  other  furnishing 


Fig.  9. — Thermograph  in  special  Fruit  Frost  Service  shelter. 


a  cover  for  the  thermometer.  The  cover  is  hinged  so  the  indicator 
can  be  set,  which  is  done  by  elevating  the  bulb  end  of  the  instrument. 
Figure  8  shows  a  minimum  thermometer  in  a  simple  shelter.  The 
standard  exposure  is  at  a  height  of  4%  to  5  feet  from  the  ground 
with  the  shelter  facing  north  so  as  to  prevent  the  sun  from  striking 
any  part  of  the  thermometer.  The  best  thermometers  of  this  type 
are  graduated  to  a  short  range  and  may  be  injured  by  extreme  heat. 
During  the  summer  they  should  be  removed  from  the  shelters  and 
stored  with  the  bulb  ends  down,  in  a  cool  place. 


BULL.  398]  ORCHARD    HEATING    IN    CALIFORNIA  41 

An  open  flame  from  match  or  torch  should  never  be  used  to  read 
a  thermometer.  The  only  safe  light  is  that  from  an  electric  flashlight. 
The  reading  should  be  made  quickly  and  the  observer  must  take  care 
not  to  breathe  on  the  bulb  of  the  thermometer. 

If  the  minimum  thermometer  is  roughly  handled  or  improperly 
exposed  the  column  of  liquid  may  separate.  The  thermometer  can 
sometimes  be  put  back  into  condition  by  attaching  a  three-foot  length 
of  stout  string  to  the  top  end  and  whirling  it  rapidly.  Repairs  should 
be  made  by  a  Weather  Bureau  representative  if  possible. 

The  U  tube  type  of  thermometer  which  reads  both  maximum  and 
minimum  is  very  likely  to  be  out  of  order  and  is  not  recommended 
for  use  in  connection  with  orchard  heating. 

Many  growers  have  thermographs  so  that  they  may  have  an  exact 
record  of  the  temperatures  they  have  maintained  in  the  orchards. 
The  temperature  record  charts  shown  in  figures  11,  12  and  13  are 
taken  directly  from  orchard  thermograph  records. 

A  thermograph  properly  exposed  in  a  special  Fruit  Frost  Service 
instrument  shelter  is  shown  in  figure  9.  The  thermograph  should  be 
checked  daily  against  a  tested  thermometer  in  the  same  shelter. 

In  districts  where  there  are  no  frost  warning  patrols,  frost  alarms 
are  frequently  used.  In  order  to  be  safe  they  should  be  of  the  type 
that  rings  when  the  circuit  is  broken,  rather  than  when  a  contact 
is  made.  If  anything  goes  wrong  with  the  line  the  bell  rings.  This 
will  occasionally  result  in  the  grower  being  called  too  soon. 

The  alarm  should  be  tested  each  night  during  the  danger  season 
by  pulling  the  switch.  These  alarms  are  set  for  28°  F.,  30°  F.,  or  32°  F. 
They  should  be  exposed  in  proper  shelters  in  the  coldest  locations. 


ORCHARD    HEATING    METHODS 

Complete  success  in  orchard  heating  is  not  attainable  without  an 
adequate  number  of  heaters  of  sufficient  fuel  capacity  and  proper 
accessory  equipment.  Equipment  itself,  however,  merely  renders 
success  possible  but  does  not  assure  it.  The  individual  whose  per- 
sonal efficiency  is  high  may  save  his  crop  with  rather  inferior 
equipment  while  his  neighbor  with  much  better  equipment  may  fail. 
The  essential  of  success  is  sufficient  heat  at  the  proper  time.  To 
provide  the  necessary  heat  the  grower  must  be  fully  prepared  at  all 
times  during  the  period  of  possible  danger  and  must  understand  how 
to  handle  the  firing  under  his  own  conditions  with  the  type  of  heaters 
at  his  disposal.    In  this  section  suggestions  are  offered  as  to  operating 


42 


UNIVERSITY    OF    CALIFORNIA- — EXPERIMENT    STATION 


methods  but  each  grower  must  learn  how  to  solve  his  local  problem 
by  actual  experience  in  heater  operation.  An  excellent  precautionary 
practice  is  the  lighting  of  a  few  heaters  after  they  are  in  the  field 
and  supposedly  ready  for  emergency  use.  It  is  especially  advisable 
to  burn  new  heaters  an  hour  or  two  as  a  means  of  getting  them  in 
condition  to  light  readily  later. 


.:.'-' 


Fig.   10. — Citrus  orchard  banked  with  heaters   on  the  windward  side;    one  large 

capacity  heater  per  tree. 

For  Citrus  orchards  heaters  should  be  placed  in  the  field  not  later 
than  November  15th  and  for  deciduous  orchards  well  in  advance  of 
the  first  indications  of  swelling  of  the  buds.  The  heaters  should  be 
placed  so  as  to  give  a  uniform  distribution  of  heat.  It  is  advisable 
to  have  a  row  of  heaters,  one  to  a  tree,  outside  of  the  orchard  on  the 
side  from  which  the  air  is  drifting.  Figure  10  shows  an  orange 
orchard  banked  with  one  heater  of  large  capacity  outside  each  tree. 
In  some  districts  it  is  advisable  to  bank  two  sides  in  this  way  and  in 
cold  locations  two  outside  rows  may  need  to  be  banked  with  a  heater 
to  each  tree.  As  pointed  out  previously,  the  orchard  heating  problem 
consists  in  part  of  replacing  the  heat  lost  to  the  drifting  air  but  mainly 
in  making  up  for  losses  from  radiation.  Inasmuch  as  radiation  occurs 


BULL.  398]  ORCHARD   HEATING   IN    CALIFORNIA  43 

uniformly  throughout  the  orchard,  the  ideal  manner  of  lighting  the 
heaters  would  be  to  leave  no  dark  or  unlighted  rows.  For  this  reason 
it  is  believed  that  it  is  better  to  have  a  heater  to  every  other  tree  in 
every  row  than  to  place  them  one  to  a  tree  in  alternate  rows.  Con- 
venience and  speed  in  firing  and  ease  of  filling  must  be  taken  into 
consideration,  however,  in  placing  the  heaters.  When  the  heaters 
have  been  placed  in  the  field  the  thermometers  should  be  set  up,, 
torches  filled,  and  placed  together  with  a  reserve  supply  of  torch 
fuel  in  a  convenient  location. 

Accurate  information  concerning  weather  conditions  is  helpful 
in  determining  firing  plans.  The  forecasts  issued  by  the  Weather 
Bureau  every  evening  where  a  local  Fruit  Frost  Service  representa- 
tive is  available,  include  an  estimate  of  the  minimum  temperature 
likely  to  be  reached  at  a  certain  key  station,  information  as  to  the 
probable  dew  point,  wind  conditions  and  the  amount  of  temperature 
inversion.  A  forecast  of  this  kind  provides  the  grower  with  exactly 
the  information  necessary  for  determining  approximately  how  diffi- 
cult the  heating  problem  is  likely  to  be  for  any  given  night.  If  local 
Fruit  Frost  Service  forecasts  are  not  available,  special  evening 
forecasts  may  sometimes  be  had  from  a  district  office  of  the  Weather 
Bureau.  The  county  farm  advisor  should  be  consulted  concerning 
the  frost  forecasting  service  available  in  the  district. 

If  a  severe  and  early  drop  of  temperature  is  expected  lighting 
should  begin  at  a  higher  temperature  than  if  the  duration  of  cold 
is  expected  to  be  only  for  a  short  time.  If  the  temperature  drop  has 
been  rapid  and  occurs  late  in  the  night  the  fruit  temperature  will 
lag  behind  the  air  temperature  and  lighting  may  be  delayed  some- 
what.    (See  section  on  damaging  temperatures.) 

Lighting  Heaters.  The  danger  point  as  explained  in  another 
section  varies  according  to  the  type  and  severity  of  the  frost  expected 
but  when  it  is  reached  the  grower  should  take  steps  to  make  heat 
available  at  once  over  the  entire  acreage.  The  first  heaters  lighted 
should  be  the  border  rows,  especially  on  the  windward  side,  and  then 
about  one-fourth  of  the  heaters  throughout  the  orchard.  It  it  better 
to  light  one-fourth  of  the  heaters  in  each  row  rather  than  all  of 
the  heaters  in  every  fourth  row.  Periodic  inspections  of  the  ther- 
mometers should  be  made  and  if  the  temperature  continues  to  drop 
more  heaters  should  be  lighted,  this  operation  being  repeated  as  often 
as  may  be  necessary.  The  greatest  economy  in  fuel  usage  may  be 
obtained  by  maintaining  the  temperature  just  above  the  danger  point 
rather  than  by  allowing  it  to  fluctuate  greatly.  This  is  accomplished 
in  two  ways,  first  by  varying  the  number  of  heaters,  burning  per  acre 


44  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

and  second,  by  controlling  the  rate  of  fuel  consumption  through 
heater  regulation.  When  a  marked  fall  in  temperature  occurs  the 
most  efficient  way  to  increase  the  heat  is  to  light  more  heaters  and 
regulate  all  of  them  to  a  moderate  fire.  If  still  more  heat  is  required 
after  all  of  the  heaters  are  lighted  it  may  be  provided  by  opening 
the  drafts  and  increasing  the  rate  of  burning.  This  procedure  is  in 
harmony  with  the  basic  principle  of  a  relatively  large  number  of 
small  fires  per  acre  which  provides  for  the  most  uniform  distribution 
of  the  heat  generated. 

The  thermograph  records  from  heated  and  unheated  orchards 
shown  in  figures  11*,  12*  and  13*  illustrate  satisfactory  as  well  as 
unsatisfactory  temperature  control.  Figure  11  shows  superimposed 
thermograph  records  from  instruments  located  in  neighboring  heated 
and  unheated  navel  orange  orchards  the  same  night.  The  solid  line 
shows  the  temperature  in  the  unprotected  grove  and  the  dotted  line 
the  temperature  record  for  a  grove  protected  with  fifty  7-gallon  oil 
heaters  per  acre.  The  following  facts  should  be  noted:  (1)  The  heat 
from  the  border  rows  of  heaters  merely  checked  the  fall  in  tem- 
perature of  the  heated  orchard  with  the  outside  temperature  steadily 
falling;  (2)  the  burning  of  twenty-five  heaters  per  acre  from  10  p.m. 
until  1  a.m.  did  not  maintain  a  safe  temperature  even  though  these 
heaters  were  burned  at  the  maximum  rate;  (3)  fifty  fires  per  acre 
did  maintain  a  satisfactory  temperature  as  long  as  they  were  kept 
burning;  (4)  failure  to  provide  sufficient  field  fuel  capacity  by 
having  heaters  of  larger  size  or  some  unlighted  heaters  in  reserve, 
coupled  with  the  waste  of  oil  early  in  the  night  from  improper  regu- 
lation was  responsible  for  the  severe  drop  in  temperature  starting 
at  4:15  a.m.  when  the  first  lighted  heaters  burned  dry.  The  loss  of 
fruit  in  this  orchard  was  not  great  but  if  fifty  properly  regulated 
heaters  had  been  kept  burning  from  9  :30  p.m  until  8  a.m.  a  much 
more  satisfactory  temperature  control  would  have  been  obtained. 

Figure  12  shows  superimposed  thermograph  records  from  neigh- 
boring pear  orchards,  one  protected  with  small  lard  pail  type  heaters 
and  the  other  unprotected.  The  records  were  taken  during  the  night 
of  April  13-14,  1919,  when  the  temperature  inversion  was  only  3°  F. 
in  35  feet  and  heating  conditions  were  difficult.  It  will  be  noted 
that  with  no  other  regulation  than  an  increase  in  the  number  of 
heaters  burning  per  acre  a  satisfactory  temperature  was  maintained 
all  night  with  the  exception  of  a  few  minutes  following  4:40  a.m. 
when  the  first  lighted   heaters  burned   dry.     Reserve  heaters   were 


*  Young,   Floyd   D.,   and   Cate,   C.    C.      Damaging   temperatures   and   orchard 
heating  in  the  Rogue  River  Valley,  Oregon,  Mo.  Weather  Eev.  51:617-631.    1923. 


Bull.  398] 


ORCHARD    HEATING    IN    CALIFORNIA 


45 


Temperature  f'F) 
^  ^  <-> 

0>  <*>  <!> 


1 

1 

i 

1 

1 

1 

^ 

^x^ 

Sill 
1   Temperature  in  pro - 
>  /«:/«/  ^/we  before 
J  heaters  tvere  lighted 

yf  — 

^Outside  b 

order  routs _ 
s  burning 

<fr 

'of  heater 

v^V0'' 

I 

I* 

< 

'    —          1 

j 

1 

/ 

/ 
( 

.  One -half  of  heaters 
burning  (IS  to  acre) 

V 

f 
1 
/ 

< 

\.       .at 

\. 

n 

c 

s 

.  50  heate 
Z_acre  out 

rs  to  the 
'ning 

ll 

( 

> 

f 

.first tight 

ing  of  heat- 
ing out  — 

J 

^ 

(} 

J 

V 

{ 
> 
\ 

1 

.  One-hah 
'  burning 

'  of  heaters 
f25foacre) 

^ 

V 

> 

"v.^ 

-  1 

1 

1" 

1 

1 

1 

\ 

\ 

Fig.  11. — Thermograph  records  for  the  same  night  from  neighboring  heated  and 
unheated   Navel   orange    orchards.* 


*  Young,   Floyd   D.     Notes   on   the   1922   freeze   in   southern   California.      Mo. 
Weather  Eev.  51:584.    1923. 


46 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


lighted  until  a  total  of  fifty-four  per  acre  were  burning.  Economy 
of  fuel  usage  was  obtained  by  keeping  the  temperature  just  above 
the  danger  point. 

This  pair  of  records  should  be  contrasted  with  another  pair 
(figure  13)  obtained  in  two  pear  orchards  on  the  night  of  May  4-5, 
1919,  when  weather  conditions  were  almost  ideal  for  heating  and  the 
temperature  inversion  was  about  twice  as  great  as  on  the  night  of 
April  13-14.  The  burning  of  thirty-six  heaters  per  acre  of  the  five- 
quart  lard  pail  type  rapidly  raised  the  temperature  several  degrees 


11  p.  m 


8  a.  m. 


Fig.    12. — Temperature   records   in   a  heated   orchard   and   at   an   outside   check 
station  on  the  night  of  April  13-14,  1919. 

above  the  danger  point.  Less  oil  would  have  been  consumed  if  the 
grower  had  lighted  only  nine  heaters  per  acre  at  the  first  firing  and 
then  more  if  necessary,  as  was  done  the  night  of  April  13-14. 

If  the  grower  is  to  maintain  a  safe  temperature  with  the  minimum 
fuel  consumption  it  must  be  done  by  intelligent  firing  based  on  a 
careful  check  of  actual  temperatures  at  frequent  intervals. 

Extinguishing  Heaters.  The  time  for  extinguishing  heaters  should 
be  determined  from  the  temperature  shown  by  the  check  thermometer 
situated  outside  the  heated  area.  The  temperature  is  frequently  below 
the  danger  point  for  an  hour  or  more  after  sunrise,  and  the  fires 
should  not  be  put  out  too  soon.  It  is  necessary  to  keep  briquet  heaters 
refueled  up  to  sunrise  even  though  they  cannot  be  put  out  and  some 


Bull.  398] 


ORCHARD    HEATING    IN    CALIFORNIA 


47 


loss  of  fuel  will  be  inevitable.     All  types  of  heaters  are  best  extin- 
guished by  closing  the  drafts  tightly  and  capping  the  stacks. 

Refilling.  Refilling  should  begin  as  soon  as  the  heaters  are  com- 
pletely extinguished  and  should  continue  until  all  have  been  filled. 
Many  losses  have  occurred  through  failure  to  refill  after  each  night 
of  burning,  even  though  only  a  small  part  of  the  oil  had  been  burned. 
If,  for  any  reason,  refilling  is  not  completed,  the  full  heaters  from 


10  p.  m.      11  p.m. 


40c 


mdt 


1  a.  m.       2  a.  m.      3  a.  m.     4  a.  m.      5  a.  m.      6  a.  m.      7  a.  m. 


38° 


36c 


34° 


32c 


30° 


28° 


26c 


1           1           1           1           1 

I    V 

V  \ 

Temperature  in  fired  orchard 

(heaters  not  burning) 

36  Heaters  to  acre  burning 

\\  \ 

,     putting  out  all    heaters 

\\ 

\  V 

VA 

\> 

V 

r» 

'\v    I 

s 

\  1 

\ 

V 

\ 

. 

> 

^^^ 

#* 

\\ 

M 

i 

\a 

^\ 

. 

v  \ 

\\ 

• 

\ 

V.' 

v--> 

• 

\ 

* 

V 

v'~\> 

V*" 

s 

Yig.    13. — Temperature   records    in   a   heated    orchard    and    at   an   outside    check 
station  on  the  night  of  May  4-5,  1919. 

the  reserve  of  the  previous  night  should  be  lighted  first  the  next  night, 
so  when  the  most  fires  are  needed,  usually  about  4  or  5  a.m.,  there 
will  be  some  fuel  in  all  of  the  heaters.  At  the  time  of  filling,  all 
lighting  cups  in  stacks  or  drafts  should  be  primed  with  heater  oil. 
Great  care  should  be  taken  to  keep  oil  from  being  spilled  on  the 
ground,  as  it  will  kill  roots  with  which  it  comes  in  contact. 

Labor  for  Operations.  One  man  can  light  2y2  to  5  acres,  depend- 
ing on  the  type  of  heaters  and  the  amount  of  regulating  required. 
Fruit  pickers  and  high  school  and  college  students  furnish  the  usual 
source  of  labor  for  orchard  heating  operations. 


48  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


Lighting  Instructions  for  Different  Types  of  Heaters 

Lard  Pail  Type.  Remove  the  cover  and  pour  a  ring  of  fire  from 
the  torch  around  the  edge  of  the  heater.  After  the  heaters  have 
been  burned  once  they  will  light  easily  with  a  few  drops  of  burning 
torch  fuel  because  the  crust  of  soot  around  the  rim  of  each  heater 
serves  as  a  wick. 

Low  Stack  Heaters.  Dunn  heaters  or  Scheu  "Double  Stack" 
heaters  should  be  wicked  with  a  small  amount  of  excelsior  in  the 
down-draft  tube,  especially  when  new  or  clean.  They  can  be  lighted 
with  one  draft  hole  open.  They  should  be  lighted  by  pouring  burning 
torch  fuel  around  the  inside  of  the  stack  or  in  the  stack  lighting  cup 
and  also  through  the  draft.  New  low  stack  heaters  will  light  more 
easily  if  they  are  burned  an  hour  or  two  when  first  placed  in  the 
orchard. 

In  order  to  keep  heaters  of  this  type  burning  satisfactorily,  they 
should  be  regulated  two  or  three  times  during  the  night  and  as  the 
oil  burns  low  the  draft  must  be  increased.  If  the  drafts  are  open  too 
wide,  the  amount  of  smoke  produced  is  excessive.  This  is  true  of 
nearly  all  heaters. 

The  foregoing  instructions  apply  to  the  "Citrus"  heater  except 
that  wicking  should  be  placed  in  the  stack  instead  of  the  draft.  This 
is  very  necessary  with  new  heaters  being  burned  for  the  first  time. 
Pieces  of  palm  leaf,  cornstalks,  or  eucalyptus  bark  placed  in  the 
reservoir  and  sticking  up  into  the  stack  will  serve  as  satisfactory 
wicks.  Heater  bowl  covers  should  fit  tightly  onto  the  fuel  reservoirs 
so  that  all  air  entering  the  combustion  chamber  will  have  to  pass 
through  the  draft  openings. 

Medium  and  Tall  Stack  Heaters  with  Down-draft  Tubes  or  Plates. 
These  heaters  must  be  set  level  and  all  models  previous  to  1925  are 
hard  to  light  if  filled  too  full.  The  1925  models  will  hold  ten  gallons 
but  the  same  size  bowl  on  previous  models  will  not  work  satisfactorily 
with  more  than  nine  gallons.  To  light  these  heaters,  the  draft  covers 
should  be  thrown  back  and  burning  torch  fuel  poured  around  the 
outside  of  the  stack  and  into  the  draft.  Pouring  the  torch  fuel  around 
the  stack  assists  in  starting  an  upward  draft  and  insuring  lighting. 

One  man  should  light  and  another  should  follow,  five  minutes 
behind,  closing  the  draft  covers  and  regulating  the  holes  so  that  the 
tip  of  the  flame  barely  emerges  from  the  stack.  The  efficiency  of 
heaters  of  this  type  is  greatest  with  this  sort  of  regulation. 

If  the  draft  deflector  is  of  the  fluted  plate  type  it  must  slant  toward 
the  center  of  the  bowl  from  the  top  down.     If  V-type  deflectors  are 


BULL.  398]  ORCHARD   HEATING   IN    CALIFORNIA  49 

used,  the  open  side  of  the  V  must  be  next  to  the  side  of  the  bowl,  or 
away  from  the  stack.  The  lighting  of  heaters  with  these  two  latter 
types  of  deflectors  is  facilitated  by  wicking  with  shook  rope  or  other 
material  hung  down  in  the  draft  hole. 

If  the  heater  is  too  full  or  regulated  too  soon  after  lighting  it  is 
likely  to  go  out.  Heaters  of  this  type  should  be  regulated  several 
times  during  the  night  to  keep  the  flame  tip  coming  out  of  the  stack. 
The  stacks  should  be  desooted  when  refilling,  if  necessary. 

Separate  Reservoir  Types.  The  Kittle  heater  should  be  lighted 
by  pushing  the  handle  one-third  of  the  way  down  and  pouring  burning 
torch  fuel  at  the  base  of  the  stack.  Regulation  is  accomplished  by 
pushing  the  handle  down  further  every  three  or  four  hours.  The 
vacuum  feed  types  of  heaters  are  automatic  in  feed,  the  rate  of  flow 
being  controlled  by  the  draft  regulation.  They  are  lighted  in  the 
draft  and  down  or  around  the  stacks.  The  valve  feed  types  are  lighted 
by  turning  on  the  valve  and  lighting  in  the  burner  or  stack.  The  rate 
of  flow  varies  as  the  oil  warms  up  and  the  viscosity  changes;  this 
type  of  heater  therefore  requires  frequent  regulation  for  best  results. 

Briquet  Heaters.  These  are  easily  lighted  if  proper  kindling  is 
used.  They  should  be  filled  nearly  full,  the  kindling  put  in  and  then 
a  few  more  briquets  added.  They  are  lighted  from  the  top  with 
burning  torch  fuel.  Oil-soaked  shavings  or  shaving  briquets,  pieces 
of  automobile  tire  about  two  to  three  inches  long  cut  across  the  casing, 
or  a  large  handful  of  peach  pits  are  satisfactory  kindling  materials. 

Coal  briquets  can  be  lighted  without  kindling  if  previously  primed 
with  smudge  oil.  This  should  be  done  after  the  briquets  are  in  the 
heaters  and  several  days  in  advance  of  the  period  when  damaging 
temperatures  are  expected.  A  good  kindling  for  carbon  briquets  can 
be  made  by  soaking  part  of  the  briquets  in  a  light  grade  of  oil,  known 
as  saw  oil,  and  adding  three  of  these  to  the  charge  for  each  heater. 
These  will  retain  enough  oil  to  ignite  satisfactorily  after  several  weeks. 
Briquet  heaters  require  little  or  no  regulation  but  should  be  refueled 
every  P/2  to  2  hours  with  only  one  layer  of  fuel  each  time  so  as  not  to 
depress  the  fire  too  much  by  the  addition  of  cold  fuel. 

General  Directions.  Lighting  heaters  is  a  more  difficult  task  on 
a  cold  night  than  on  a  warm  day.  If  the  night  is  damp  the  covers 
may  be  frozen  on  and  the  drafts  frozen  shut  when  lighting  should 
begin.  Pliers  and  draft  tools  should  always  be  at  hand.  Some 
growers  with  a  large  acreage  to  heat  remove  the  caps  and  open  the 
drafts  before  night,  if  the  forecasts  and  general  weather  conditions 
indicate  the  probability  of  a  need  for  rapid  lighting. 


50  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


CARE  OF   ORCHARD    HEATERS 

When  the  extreme  conditions  of  moisture  and  temperature  which 
orchard  heaters  must  undergo  in  their  use  and  storage  are  considered, 
the  average  rate  of  depreciation  is  surprisingly  low,  being,  in  fact, 
much  less  than  was  anticipated  when  oil-burning  heaters  first  came 
into  use  on  a  commercial  scale.  Many  heaters  have  been  in  use  in  citrus 
orchards  for  ten  or  twelve  years  and  are  still  in  reasonably  good 
condition,  even  though  comparatively  little  care  has  been  given  to 
them.  The  high  cost  of  heating  equipment  makes  it  desirable  to 
extend  their  period  of  service  as  much  as  possible. 

The  essential  points  in  the  care  of  orchard  heaters  are  to  prevent 
rusting  and  to  keep  them  relatively  free  from  the  accumulations  of 
soot  and  residue  which  reduce  their  fuel  capacity  and  burning 
efficiency. 

The  effective  life  of  heaters  may  be  prolonged  appreciably  if, 
immediately  after  delivery  and  before  placing  in  the  orchard,  the 
bowls  or  reservoirs  are  clipped  in  asphaltum  paint  or  some  similar 
compound  which  will  protect  the  metal  against  rust.  As  the  efficiency 
of  this  coating  diminishes,  it  is  advisable  to  renew  it ;  this  is  generally 
done  at  the  time  of  removing  the  residue.  As  an  additional  means 
of  preventing  rusting  of  the  bowls,  many  growers  use  some  sort  of 
support  intended  to  keep  the  heaters  from  resting  directly  on  the 
ground  when  in  the  orchard.  Tar-paper  squares,  wooden  platforms, 
small  cleats,  and  bricks  have  been  used  for  this  purpose.  Since,  if  the 
heater  is  lighted  at  all,  the  bowl  cover  and  stack  usually  reach  a 
temperature  which  will  destroy  the  efficiency  of  whatever  compound 
is  used  to  prevent  rusting,  there  seems  to  be  no  particular  reason  for 
using  expensive  materials  for  coating  these  parts.  Dipping  them  in 
heavy  oil,  such  as  the  sludge  remaining  at  the  end  of  the  season, 
appears  to  be  as  satisfactory  a  treatment  as  any.  This  should  be  done 
every  season,  and  especially  in  the  case  of  the  parts  of  the  stack  which 
are  most  subject  to  rust.  Such  treatment  may  be  given  in  the  orchard 
or  at  a  central  point,  in  case  the  heaters  are  removed  from  the  orchard 
for  cleaning  and  storage. 

Cleaning  the  accumulations  of  soot  and  residue  from  the  heaters 
presents  a  problem  of  special  importance,  since  if  this  is  not  done 
from  time  to  time  their  fuel  capacity  and  burning  efficiency  may 
become  seriously  reduced.  The  accumulation  of  asphaltum  resulting 
from  a  prolonged  period  of  burning  of  the  distilling  types  of  heaters 
may  reduce  the  fuel  capacity  as  much  as  25  per  cent  and,  therefore, 


BULL.  398]  ORCHARD   HEATING   IN    CALIFORNIA  51 

automatically  renders  it  difficult  to  secure  as  much  heat  after  six  or 
eight  hours  of  burning  as  may  be  necesary  to  save  the  crop.  Such 
accumulations  also  increase  the  difficulty  of  relighting  the  heaters  in 
case  this  must  be  done  before  they  are  refilled.  The  heaters  should 
be  cleaned,  therefore,  as  often  as  may  be  necessary,  which  depends 
on  the  length  of  time  they  have  been  burned  and  on  the  quality  of 
fuel  used. 

Methods  of  cleaning  vary  greatly  and  must  be  determined  in 
large  part  by  the  conditions  under  which  the  heaters  are  operated. 
Some  growers  have  attempted  to  clean  heaters  by  burning  them  dry 
in  the  orchard.  With  some  types  of  heaters,  such  as  the  lard-pail 
types,  this  is  impossible,  because  the  draft  is  insufficient.  Lard-pail 
heaters  are  generally  used  over  so  short  a  period,  however,  that  accu- 
mulations of  residue  are  small  and,  therefore,  not  particularly  objec- 
tionable. It  is  possible  to  burn  most  of  the  distilling  types  dry ;  the 
charred  and  caked  residue  may  then  be  removed  by  jarring  the 
bowls.  Burning  dry  must  be  done  with  caution,  in  the  case  of  the 
down-draft  types,  because  the  heat  generated  may  be  sufficient  to 
warp  the  bottoms  and  produce  rapid  deterioration.  The  system  of 
burning  the  residue  out  at  a  central  point  by  the  use  of  distillate, 
which  was  formerly  used  somewhat,  has  been  practically  abandoned. 
If  the  heaters  are  stored  under  the  trees,  which  many  growers  do, 
a  common  practice  is  to  empty  them  and  clean  them  thoroughly  with 
scraping  tools  designed  for  that  purpose,  before  storing  them.  A 
special  cleaning  crew  of  two  or  three  men  and  a  team  and  wagon 
equipped  with  sludge  tank  and  platform  for  scraping  and  dipping 
the  covers  and  stacks,  is  usually  employed  for  this  purpose.  The 
heaters  are  then  placed  under  the  trees,  where  they  remain  until  fall. 
In  seasons  when  the  heaters  have  not  been  burned  much,  many  growers 
omit  the  emptying  and  cleaning  operations  and  store  the  heaters  full 
of  oil  under  the  trees.  Where  this  is  done  there  is  the  possibility  of 
damage  from  oil  seeping  out  or  being  spilled  from  the  heaters  and 
killing  some  of  the  tree  roots.  This  may  be  minimized  by  carefully 
inspecting  the  heaters  at  the  time  they  are  placed  under  the  trees, 
and  repairing  all  leaky  heaters. 

When  the  accumulation  of  residue  becomes  large,  such  as  occurs 
following  a  season  of  prolonged  burning,  or  at  intervals  of  several 
years  of  ordinary  use,  most  growers  find  it  advisable  to  assemble 
the  heaters  at  a  central  point,  where  they  can  be  thoroughly  cleaned 
and  coated  with  suitable  rust-preventing  compounds.  The  procedure 
usually  followed  is  to  empty  the  heaters  and  haul  them  to  an  open 
place  on  waste  land,  preferably  in  a  dry  river  bed,  which  permits 


52  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

of  easy  disposal  of  the  sludge.  Some  means  of  heating  the  bowls  in 
batches  of  a  dozen  or  more  at  a  time  is  provided.  A  trench  covered 
with  sheet  iron  and  with  a  flue  at  one  end,  is  often  used  for  this  pur- 
pose. The  fuel  used  is  usually  the  sludge  oil  left  in  the  heaters  after 
emptying.  When  the  bowls  are  hot  they  are  picked  up  by  means 
of  tongs,  and  the  residue  shaken  out.  They  are  then  scraped  or 
brushed  with  steel  brushes,  tested  for  leaks  by  immersing  in  kero- 
sene or  distillate,  repaired  if  necessary,  and  dipped  in  hot  asphaltum 
paint.  After  drying,  the  bowls  and  covers  are  usually  nested  together 
and  the  stacks  placed  in  piles.  Some  growers  store  them  under  cover, 
others  merely  stack  them  on  well-drained  land  close  to  the  orchard. 

Repairing  of  heaters  consists  principally  in  stopping  leaks  and  in 
straightening  out  the  few  bowls  and  covers  which  are  bent.  The 
average  grower  does  not  attempt  to  do  this  work  himself  but  sends 
such  heaters  to  a  tinsmith  to  be  repaired.  Some  of  the  larger  Citrus 
fruit  producing  companies  have  found  it  profitable  to  repair  their  own 
heaters  on  account  of  the  large  number  they  have  to  use.  Holes  are 
repaired  where  possible  by  means  of  soft  copper  rivets  or  by  welding 
or  brazing.  Soldering  has  been  used  to  some  extent  but  the  solder 
melts  if  it  comes  in  contact  with  the  flames. 

There  is  wide  variation  among  growers  with  respect  to  storage  of 
heaters  during  the  period  when  they  are  not  in  use.  The  principal 
factor  determining  the  place  and  method  of  storage  appears  to  be 
the  system  of  handling  the  heaters  which  is  followed.  Some  growers 
take  the  heaters  apart,  clean  and  dip  them  and  store  them  under 
cover  every  year.  This  is  probably  more  care  than  is  necessary  under 
most  conditions.  Others  go  to  this  trouble  only  when  residue  accu- 
mulates to  the  point  where  the  heaters  must  be  cleaned,  storing  them 
under  the  trees  or  alongside  the  orchard  during  the  periods  between 
cleanings.  Some  store  them  under  the  trees  full  of  oil,  while  others 
store  them  empty. 

No  blanket  recommendations  can  be  offered  concerning  methods 
of  care  of  orchard  heaters.  These  must  of  necessity  be  determined 
by  each  grower  for  himself  in  accordance  with  the  type  of  heaters  he 
has,  the  amount  of  burning  required,  the  humidity  and  rainfall  in  the 
district,  and  the  most  economical  and  efficient  method  of  handling 
them  in  his  orchard. 

With  good  care  the  depreciation  of  heaters  should  not  be  excessive, 
the  bowls  and  covers  lasting  from  ten  to  fifteen  years,  and  the  stacks 
from  three  to  five  years. 


BULL.  398]  ORCHARD   HEATING   IN    CALIFORNIA  53 

TEMPERATURES   AND    CONTRIBUTING    CONDITIONS    CAUSING 

FROST   DAMAGE 

So  many  factors  must  be  taken  into  consideration  in  determining 
whether  a  given  temperature  will  cause  damage  to  fruits,  buds,  or 
blossoms,  that  the  matter  is  one  of  considerable  complexity.  The 
length  of  time  the  low  temperature  persists,  the  vigor  of  the  tree,  and 
the  weather  preceding  the  frost,  all  have  considerable  influence  on 
the  amount  of  damage  that  will  be  done.  If  the  sky  clouds  over  before 
sunrise  on  a  frosty  night,  so  that  the  direct  rays  of  the  sun  do  not 
strike  the  trees  until  after  the  ice  is  out  of  the  fruit,  the  damage 
is  likely  to  be  lessened.  Other  conditions  being  the  same,  the  fruit  or 
blossoms  on  a  weak,  undernourished  tree  will  show  more  injury  than 
those  on  a  vigorous,  healthy  tree,  after  both  have  been  subjected  to 
the  same  low  temperature. 

When  soil  and  atmospheric  conditions  are  favorable  for  growth, 
which  may  be  the  case  during  warm,  sunshiny  weather,  the  sap  is 
likely  to  be  watery  and  its  freezing  point  high.  For  this  reason  a 
frost  which  follows  a  period  of  weather  favorable  for  rapid  growth 
will  cause  more  damage  than  the  same  temperature  following  a 
period  of  cold,  cloudy  weather  and  consequent  slow  growth.  Under 
certain  conditions  the  blossoms  and  fruit  may  endure  temperatures 
without  damage  which  under  other  conditions  would  destroy  the 
greater  portion  of  the  crop.  In  the  following  paragraphs  data  are 
given  regarding  temperatures  which  have  caused  damage  to  various 
deciduous  and  Citrus  fruits  in  different  stages  of  growth.  These  data 
are  based  on  field  observations  made  by  weather  bureau  officials 
over  a  long  period  of  years  and  are  considered  safe  as  a  basis  of 
recommendations  for  successful  heating  operations.  However,  long 
experience  is  the  best  guide  in  deciding  when  to  light  the  orchard 
heaters.  Some  fuel  will  be  wasted  in  maintaining  temperatures  accord- 
ing to  these  recommendations,  but  the  grower  who  has  orchard  heating 
equipment  cannot  afford  to  take  chances.  All  temperatures  men- 
tioned are  in  degrees  Fahrenheit  as  registered  on  sheltered  ther- 
mometers. 

Temperatures  Damaging  to  Deciduous  Fruits.  Table  11  gives  the 
temperatures  as  registered  by  sheltered  thermometers  which  will  be 
endured  for  thirty  minutes  or  less  by  deciduous  fruits  in  various 
stages  of  development. 

It  must  be  admitted  that  the  data  in  the  following  table  are  unsatis- 
factory because  they  do  not  go  enough  into  detail.  The  three  stages 
of  development  given  are  not  sufficient  to  cover  all  the  changes  in 


54  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

susceptibility  to  damage  which  are  found  during  the  period  of  devel- 
opment of  a  fruit  bud  through  the  blossom  stages  to  the  green  fruit. 
Detailed  information  to  supply  the  deficiency  is  not  available  except 
in  the  case  of  a  few  fruits. 

TABLE  11 

Temperatures  Endured  for  Thirty  Minutes  or  Less  (Sheltered 

Thermometers)  by  Deciduous  Fruits  in  Various  Stages 

of   Development 

Stage  of  Development 

r A 

Kind  of  Fruit  Buds  closied  blit  Full  Small  green 

showing  color  bloom  fruits 

Apples    25  28  29 

Peaches  25  26  29 

Cherries  25  28  30 

Pears  25  28  30 

Plums   25  28  30 

Apricots   25  28  31 

Prunes    26  28  30 

Almonds  26  27  30 

Grapes    30  31  31 

The  fruit  buds  of  nearly  all  deciduous  fruits  are  extremely  sus- 
ceptible to  damage  during  the  period  of  from  24  to  48  hours  before 
they  open.  The  petals  are  still  folded,  but  the  flowers  are  growing 
rapidly  and  are  extremely  tender.  Buds  in  this  condition  are  often 
injured  by  temperatures  as  high  as  those  given  in  the  table  for  small 
green  fruits.  Fortunately,  most  deciduous  fruit  trees  come  into 
full  bloom  gradually,  so  that  even  if  all  the  buds  about  to  open  at 
one  time  are  killed,  the  size  of  the  crop  is  not  reduced  materially. 
Buds  of  the  Bosc  pear  often  open  almost  simultaneously,  and  a  low 
temperature  just  before  the  flowers  open  sometimes  destroys  most  of 
the  crop. 

At  the  time  generally  designated  as  "full  bloom"  most  deciduous 
fruit  trees  have  large  numbers  of  fruit  buds  which  are  still  tightly 
closed,  in  addition  to  the  flowers  which  are  fully  open.  This  makes  the 
loss  of  the  entire  crop,  or  even  the  greater  portion  of  the  crop,  on  one 
cold  night,  extremely  improbable  at  this  stage.  This  has  led  fruit 
growers  In  some  districts  to  believe  that  frost  can  do  no  damage  before 
the  fruit  has  set.  Some  growers  even  follow  the  hazardous  practice 
of  leaving  heaters  unlighted  on  frosty  nights  during  this  period. 
While  a  single  frost  at  full  bloom  seldom  affects  the  size  of  the  final 
crop,  a  series  of  heavy  frosts,  each  killing  a  portion  of  the  blossoms, 
may  leave  too  few  undamaged  buds  or  blossoms  for  a  full  crop. 

The  most  dangerous  stage  in  general  is  after  all  the  petals  have 
fallen  and  the  fruit  has  set.    All  of  the  fruit  being  in  nearly  the  same 


Bull.  398]  ORCHARD    HEATING    IN    CALIFORNIA  55 

condition,  the  entire  crop  may  be  killed  on  a  single  night.  It  is  at 
this  time  that  orchard  heating  operations  should  be  most  carefully 
conducted.  Apples  and  pears  at  this  stage  of  development  usually 
are  not  seriously  injured  by  a  temperature  of  28.5°  P.  for  thirty 
minutes  or  less,  provided  the  duration  of  temperatures  below  32°  P. 
does  not  exceed  three  hours.  If  the  temperature  drops  to  29°  P.  only  a 
short  time  before  sunrise  and  has  not  been  below  32°  P.  more  than 
three  hours,  heating  is  unnecessary.  However,  if  it  appears  that  the 
lowest  temperature  during-  the  night  will  be  below  29°  P.,  or  if  the  tem- 
perature falls  below  32°  F.  more  than  three  hours  before  sunrise, 
heaters  should  be  lighted  and  the  temperature  maintained  as  near 
31°  F.  as  possible  throughout  the  remainder  of  the  night.  Small,  green 
apricots  are  extremely  tender  just  after  the  shucks  (dried  calices)  have 
dropped  and  before  the  pits  have  hardened.  Apricots  in  this  condition 
have  been  injured  at  long  continued  temperatures  of  31°  F.,  and  many 
growers  think  it  necessary  not  to  allow  the  temperature  to  fall  below 
32°  F.  as  long  as  the  pits  are  soft. 

Different  varieties  of  the  same  fruit  often  differ  considerably  in 
their  susceptibility  to  frost  damage.  The  Delicious  apple  appears  to 
be  more  tender  than  most  other  varieties  of  apples  grown  com- 
mercially on  the  Pacific  Coast.  The  Bosc  pear  is  more  susceptible  to 
damage  by  frost  than  most  other  varieties  of  pears  at  similar  stages 
of  development,  while  the  Winter  Nelis  is  hardier  than  most  varieties. 

Temperatures  Damaging  to  Citrus  Fruits.  We  are  confronted 
with  the  same  limitations  in  attempting  to  name  definite  critical  tem- 
peratures for  Citrus  fruits  as  for  deciduous  fruits.  The  length  of 
time  the  low  temperature  persists,  the  vigor  of  the  tree,  the  weather 
preceding  the  frost,  the  maturity  of  the  fruit,  and  the  rate  at  which 
the  temperature  has  been  falling,  all  are  factors  in  determining  the 
amount  of  frost  damage  to  oranges,  lemons  and  grapefruit  that  will 
result  from  a  given  temperature.  The  size  of  the  fruits  also  is  an 
important  factor,  since  small  fruits  cool  more  rapidly  than  large  fruits. 

The  thick,  pithy  rind  of  the  orange  is  a  poor  conductor  of  heat, 
and  the  protection  it  affords  causes  the  temperature  of  the  interior 
of  the  fruit  to  fall  more  slowly  than  the  temperature  of  the  outer 
air.  When  the  air  temperature  is  falling  rapidly  the  interior  of  the 
fruit  may  be  as  much  as  seven  degrees  warmer  than  the  air  surrounding 
it,  and  the  temperature  inside  the  fruit  may  lag  from  an  hour  to  an 
hour  and  a  half  behind  the  temperature  of  the  air.  In  other  words, 
when  the  temperature  of  the  air  is  falling  rapidly,  the  air  temperature 
may  be  27°  F.  when  the  temperature  of  the  interior  of  the  fruit  is 
34°  F. 


56  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

After  the  fruit  begins  to  freeze,  its  temperature  will  remain  at,  or 
very  near,  the  freezing  point  of  the  juice  until  the  orange  is  frozen 
solid,  no  matter  how  low  the  temperature  in  the  orchard  may  fall.  The 
temperature  at  which  freezing  of  the  juice  begins  is  slightly  different 
in  different  oranges  of  the  same  variety,  even  on  the  same  tree.  In  the 
experimental  work  done  by  the  Weather  Bureau,  the  freezing  points 
of  ripe  Navel  oranges  varied  from  27°  F.  to  28°  F.  Half -ripe  Wash- 
ington Navels  began  to  freeze  at  fruit  temperatures  of  from  28°  F.  to 
29°  F,  and  green  Navels  at  from  28.5°  F.  to  29.5°  F.  The  freezing 
point  of  green  Valencia  oranges  varied  between  29°  F.  and  29.5  F. 
The  fruit  itself  must  reach  the  temperature  given  above  before  freezing 
will  begin ;  the  air  temperature  may  be,  and  usually  is,  several  degrees 
lower. 

All  objects  exposed  to  the  sky  on  a  clear  night  lose  heat  steadily 
to  the  sky  by  radiation,  and  therefore  cool  more  rapidly  than  objects 
sheltered  from  the  sky.  Citrus  fruits  on  the  outside  of  the  tree  cool 
more  rapidly  than  those  sheltered  by  foliage.  On  a  calm,  clear,  frosty 
night,  the  most  exposed  oranges  on  a  tree  may  be  as  much  as  three 
degrees  colder  than  oranges  on  the  interior  of  the  tree.  This  explains 
why  fruit  on  the  outside  of  the  tree  is  often  frozen  on  a  night  when 
sheltered  fruit  is  not  damaged.  It  often  happens  that  a  moderately 
low  temperature  will  freeze  only  the  exposed  portion  of  the  rind  of 
the  orange,  giving  it  a  water-soaked  appearance,  usually  called  the 
' '  water  mark. ' '  If  the  orange  is  colored,  the  portion  of  the  rind  that 
has  been  frozen  turns  a  very  light  yellow  on  the  day  following  the 
frost.    Such  oranges  are  called  "shiners." 

The  problem  of  determining  what  temperatures  will  damage 
oranges  is  further  complicated  by  a  phenomenon  known  as  "under- 
cooling, ' '  by  which  is  meant  the  cooling  of  the  fruit  below  the  freezing 
point  of  the  juice  without  the  formation  of  ice.  Navel  oranges  have 
been  known  to  cool  as  much  as  three  degrees  below  their  freezing 
point  before  freezing  began.  Undoubtedly  oranges  are  often  under- 
cooled  several  degrees,  after  which  the  temperature  rises  again  without 
the  formation  of  ice  and  without  any  damge  to  the  fruit  resulting. 
The  first  formation  of  ice  crystals  in  an  under-cooled  fruit  is  accom- 
panied by  a  rise  of  the  fruit  temperature  to  approximately  the  freezing 
point  of  the  juice.  The  amount  of  under-cooling  which  the  fruit  will 
undergo  on  a  given  night  cannot  be  determined  in  advance,  although 
there  appears  to  be  less  undercooling  when  the  fruit  is  covered  with 
ice  than  when  it  is  perfectly  dry.  However,  until  further  information 
regarding  under-cooling  of  fruit  on  the  tree  under  natural  conditions 
is  obtained,  it  will  not  be  practicable  to  take  this  factor  into  consider- 
ation in  determining  when  to  light  the  heaters. 


BULL.  398]  ORCHARD   HEATING   IN    CALIFORNIA  57 

In  general,  the  nights  on  which  heating  will  be  necessary  to  protect 
oranges  and  grapefruit  may  be  divided  into  two  classes.  The  first 
class  will  include  nights  on  which  a  local  frost  occurs,  when  the 
cooling  is  due  principally  to  loss  of  heat  by  radiation.  Such  nights 
usually  follow  warm  afternoons.  The  temperature  drops  rapidly,  but 
does  not  reach  27°  F.  until  2  or  3  o'clock  in  the  morning.  The  fruit 
temperature  is  likely  to  be  several  degrees  above  the  air  temperature 
on  such  nights,  and  so  long  as  the  air  temperature  continues  to  fall 
steadily,  lighting  of  heaters  to  protect  ripe,  or  nearly  ripe  Navels 
or  grapefruit  can  be  delayed  until  the  sheltered  thermometer  registers 
26°  F.  Under  similar  conditions,  heating  for  protection  of  green 
Navels  or  Valencias  should  start  at  27°  F.  In  any  case,  it  is  best  to 
keep  the  temperature  above  28°  F.  after  heating  starts. 

The  second  class  of  nights  on  which  heating  is  necessary  includes 
the  "freeze"  nights.  The  preceding  afternoons  are  usually  cold 
and  windy,  often  with  a  cloudy  sky.  The  temperature  slowly  falls 
below  the  danger  point  early  in  the  night  and  remains  there  until 
sunrise ;  or  the  temperature  may  fall  only  slightly  below  the  freezing 
point  of  the  fruit  early  in  the  night  and  remain  practically  stationary 
until  morning.  On  such  nights  it  is  necessary  to  light  the  heaters 
before  the  temperature  has  fallen  much  below  the  freezing  point  of 
the  fruit.  Lighting  of  heaters  for  nearly  ripe  Navels  should  be  started 
when  the  sheltered  termometer  reaches  27°  F.  Green  Navels  or  Valen- 
cias should  be  heated  when  the  temperature  has  been  stationary  at 
28°  F.  for  two  hours,  or  when  the  temperature  is  falling  slowly  and  has 
reached  27.5°  F.  On  a  night  when  the  temperature  falls  to  the  pre- 
viously indicated  freezing  point  of  the  fruit  before  1  a.m.,  the  tem- 
perature in  groves  protected  by  heaters  should  be  maintained  at 
this  point  until  morning. 

The  temperature  at  which  heating  is. begun  for  the  protection  of 
lemons  will  depend  on  whether  it  is  desired  to  save  the  blossoms  and 
small  "button"  lemons,  or  only  the  nearly  mature  fruit.  If  blossoms 
and  small  fruits  are  to  be  protected,  the  temperature  must  be  held 
at  30°  F.  or  higher,  while  the  larger  lemons  will  withstand  a  tempera- 
ture of  28°  F.  for  an  hour,  without  injury.  The  small  green  fruits  are 
more  susceptible  to  damage  by  frost  than  the  blossoms.  In  the  protec- 
tion of  avocados  the  general  recommendations  for  "button"  lemons 
should  be  followed. 

Damage  to  Citrus  Trees  by  Frost.  The  amount  of  injury  to  Citrus 
trees  during  a  freeze  will  depend  to  a  great  extent  on  the  weather 
preceding  the  freeze.  If  the  soil  and  air  have  been  warm,  and  there 
has  been  a  plentiful  supply  of  nitrate  available  in  the  soil  during  early 


58  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

winter,  the  trees  will  be  in  a  succulent  growing  condition  and  a  freeze 
will  cause  the  maximum  amount  of  damage.  If  the  preceding  weather 
has  been  cold  and  cloudy,  so  that  the  trees  are  semi-dormant,  damage 
by  a  freeze  will  be  considerably  less.  Trees  that  have  been  weakened 
by  lack  of  proper  care  will  be  damaged  by  higher  temperatures  than 
trees  that  have  been  kept  strong  and  vigorous.  Mature  Navel  orange 
trees  in  rather  poor  condition  were  about  75  per  cent  defoliated  by  a 
temperature  below  20°  F.  for  six  hours,  with  a  minimum  temperature 
of  18°  F.,  during  the  freeze  of  1922.  Another  Navel  orange  grove 
nearby,  in  good  condition,  was  about  10  per  cent  defoliated  by  a 
minimum  temperature  of  19.8°  F.  during  the  same  year.  On  the  morn- 
ing of  January  3,  1924,  a,  mature  orange  grove  endured  a  temperature 
of  16°  F.  for  one  and  one-half  hours,  with  thirteen  hours  below  27°  F., 
with  only  about  10  per  cent  defoliation.  In  the  last  case  the  trees  were 
almost  dormant,  while  in  1922  the  trees  had  been  in  a  growing  con- 
dition all  winter.  A  mature  lemon  grove  was  entirely  defoliated  dur- 
ing the  1922  freeze  by  a  minimum  temperature  of  20°  F.,  and  the  bark 
on  the  trunks  of  ten-year-old  lemon  trees  was  split  on  a  night  when  the 
minimum  temperature  fell  to  19°  F.  During  the  winter  of  1924-25 
several  temperature  stations  were  maintained  by  the  weather  bureau 
in  the  same  lemon  grove  of  old  trees.  In  the  lower  portion  of  the 
grove,  where  the  lowest  temperature  was  22.6°  F.,  the  trees  were  com- 
pletely defoliated.  On  slightly  higher  ground,  where  the  lowest  tem- 
perature was  24°  F.,  the  trees  were  about  50  per  cent  defoliated.  On 
still  higher  ground,  where  the  lowest  temperature  was  26.5°  F.,  only 
the  tender  new  growth  was  killed.  In  the  San  Joaquin  and  Sacra- 
mento valleys  the  trees  become  more  nearly  dormant  under  average 
winter  conditions  than  in  southern  California  and  will  consequently 
endure  somewhat  lower  temperatures  without  damage. 

Influence  of  Cover-crops  on  Orchard  Temperature.  During  recent 
years  the  belief  that  a  cover-crop  in  an  orchard  lowers  the  temperature 
several  degrees  has  become  quite  widespread.  Careful  experiments  by 
the  weather  bureau,  covering  two  entire  winter  frost  seasons,  have 
shown  that  the  temperature  is  lowered  one  degree  on  the  average,  at  a 
height  of  three  feet  above  the  ground,  by  the  presence  of  a  heavy 
cover-crop.  At  a  height  of  five  feet  above  the  ground  the  temperature 
is  depressed  only  about  one-half  degree  on  the  average.  On  nights 
when  the  temperature  barely  reached  the  danger  point  in  clean,  culti- 
vated orchards,  a  temperature  one  degree  lower  in  orchards  with  cover- 
crops  may  be  responsible  for  considerable  damage  to  fruit  within  three 
feet  of  the  ground.  In  comparison  with  the  total  production  of  fruit  in 
the  orchard,  however,  the  increase  in  the  amount  of  damaged  fruit  in 


BULL.  398]  ORCHARD    HEATING    IN    CALIFORNIA  59 

the  cover-cropped  orchard  would  be  quite  small.  The  soil  temperature, 
soil  moisture,  and  availability  of  nitrates  in  the  soil  are  factors  influ- 
encing the  state  of  dormancy  of  trees,  which  in  turn  determines  to  a 
certain  extent  the  resistance  of  both  trees  and  fruit  to  frost  damage. 
All  of  these  soil  conditions  are  influenced  by  a  growing  cover-crop 
which  may  account  for  observations  by  some  growers  that  the  cover- 
crop  is  a  benefit  and  by  others  that  it  is  a  detriment  on  frosty  nights. 


ORCHARD    HEATING    COSTS 

Costs  of  orchard  heating  vary  markedly  both  with  respect  to  the 
annual  overhead  charges,  which  are  determined  in  large  part  by 
the  initial  cost  of  equipment ;  and  also  with  operation  costs,  which 
depend  upon  the  frost  hazard  and  the  efficiency  of  operation.  Many 
growers  are  inclined  to  underestimate  the  annual  overhead  costs 
properly  chargeable  to  orchard  heating,  since  the  greater  part  of 
these  consists  of  the  two  items  of  depreciation  and  interest  on  the 
investment,  which  do  not  represent  annual  cash  outlays.  Indeed,  with 
some  growers,  it  has  been  the  practice  either  to  include  the  initial  cost 
of  orchard  heating  equipment  in  the  total  orchard  investment  or  to 
write  off  this  item  in  the  savings  made  following  a  year  of  severe  frost 
damage  and  thereafter  to  merely  charge  cash  outlays.  If,  as  previously 
stated,  however,  orchard  heating  is  to  be  regarded  as  an  investment 
which,  in  order  to  be  profitable,  must  show  a  return,  the  overhead  cost 
is  a  proper  annual  charge  and  should  include  both  interest  on  the 
capital  invested,  and  a  depreciation  charge  sufficient  to  return  the 
invested  capital  by  the  time  it  becomes  necessary  to  purchase  new 
equipment.  Although  it  has  been  shown  that  if  given  reasonable 
care  orchard  heating  equipment  depreciates  slowly,  many  growers 
feel  that  the  depreciation  charge  should  be  high  enough  to  return  the 
investment  in  a  period  of  ten  to  twelve  years,  since  advances  in 
heater  design  may  render  it  desirable  to  replace  the  equipment  before 
its  period  of  usefulness  has  been  passed. 

Initial  Cost  of  Equipment.  The  initial  cost  of  orchard  heating 
equipment  varies  in  accordance  with  the  requirements  of  the  different 
classes  of  fruits,  being  highest  with  the  lemon  and  avocado,  which, 
on  account  of  their  tenderness  and  long  fruiting  season,  require  the 
greatest  amount  of  protection,  and  lowest  with  the  temperate  zone 
fruits,  such  as  the  peach,  apricot,  and  apple,  which  arc  susceptible  to 
frost  damage  for  only  relatively  short  periods.  It  varies  also  with  the 
type  of  equipment  chosen. 


60  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Typical  cases  illustrating  cost  of  equipment  for  the  various  classes 
of  fruits  using  equipment  of  different  types  are  shown  in  Tables  12 
to  16.  Prices  of  heaters  vary  greatly  according  to  the  type  chosen. 
In  making  up  the  tables  average  prices  of  different  makes  in  the  same 
class  were  used  rather  than  actual  prices  of  any  make.  Oil  reserves 
were  calculated  on  a  safe  basis  of  three  times  the  heater  capacity.  In 
districts  where  deliveries  are  slow  or  uncertain,  larger  reserves  than 
this  will  be  necessary. 

TABLE  12 
Initial  Cost  of  Orchard  Heating  Equipment  for  Ten  Acres  of 

Oranges,  Using  "Smokeless  Heaters"  of  Large  Capacity 

500  heaters  at  $3.50  each $1,750.00 

13,500  gallons  oil  at  4c  (in  storage  tank) 540.00 

Storage  tank  (capacity  13,500  gallons) 350.00 

Wagon  and  tank  (capacity  463  gallons) 150.00 

Pipes,  connections,  buckets,  hose  and  valves  for  filling  and 

emptying  , 50.00 

Double  action  hand  pump  for  emptying 20.00 

4  thermometers  (horizontal  minimum  type).. 12.00 

4  lighting  torches    (1-gallon  capacity) - 10.00 

10  gallons  lighting  fluid  in  container - 3.00 

Total $2,885.00 

Approximate  cost  per  acre 290.00 

For  lemons  or  avocados  the  number  of  heaters  should  be  increased 

to  800  and  the  oil  storage  accordingly,  which  will  bring  the  cost  to 

approximately  $440  per  acre. 

TABLE  13 
Initial  -Cost  of  Orchard  Heating  Equipment  for  Ten  Acres  of 

Oranges,  Using  "Low  Stack"  Heaters  of  Large  Capacity 

500  heaters  at  $2.00  each $1,000.00 

13,500  gallons  oil  at  4c  (in  storage  tank)  540.00 

Storage  tank  (capacity  of  13,500  gallons) 350.00 

Wagon   and   tank 100.00 

Pipes,  connections,  buckets,  hose  and  valves  for  filling  and 

emptying  25.00 

Double  action  hand  pump  for  emptying 15.00 

3  thermometers 9.00 

3  lighting  torches 7.50 

10  gallons  lighting  fluid  in  container 3.00 

Total $2,049.50 

Approximate  cost  per  acre 205.00 

For  lemons  or  avocados  the  number  of  heaters  should  be  increased 
to  about  800  and  the  oil  storage  increased  accordingly  which  will 
bring  the  cost  to  approximately  $300  per  acre.  In  this  case  lower 
costs  than  used  in  Table  12  are  given  for  some  other  items  of  equip- 
ment as  well  as  for  the  heaters. 


BULL.  398]  ORCHARD   HEATING    IN    CALIFORNIA  61 

TABLE    14 

Initial  Cost  of  Orchard  Heating  Equipment  for  Ten  Acres  of 
Oranges,  Using  Briquet  Heaters 

1,000  heaters  at  $1.00  each $1,000.00 

60  tons  of  briquets  at  $15.00 900.00 

Trailer  for  distributing  fuel 40.00 

4  filling  funnels 10.00 

3  thermometers 9.00 

4  lighting  torches 10.00 

20  gallons  lighting  fluid  in  container 6.00 

2  tons  of  kindling  at  $15.00 30.00 

Total   $2,005.00 

Approximate  cost  per  acre 200.00 

TABLE  15 

Initial  Cost  of  Orchard  Heating  Equipment  for  Ten  Acres  of 
Deciduous  Fruits,  Using  "Lard  Pail"  Type  of  Heaters 

1,000  heaters  at  40c  each $400.00 

6,000  gallons  oil  at  6c  (in  storage  tank)  360.00 

Storage  tank   (capacity  of  6,000  gallons) 200.00 

Wagon  tank,  buckets  and  fittings 85.00 

3  thermometers 9.00 

2  lighting  torches 5.00 

10  gallons  lighting  fluid  in  container 3.00 


Total   $1,062.00 

Approximate  cost  per  acre 105.00 

TABLE  16 

Initial  Cost  of  Orchard  Heating  Equipment  for  Ten  Acres  of 
Deciduous  Fruits,  Using  Briquet  Heaters 

800  heaters  at  $1.10  each $880.00 

30  tons  of  briquets  at  $15.00 450.00 

3  filling    funnels 7.50 

3  thermometers 9.00 

3  lighting  torches  7.50 

10  gal.  lighting  fluid  in  container 3.00 

iy2  tons  kindling  at  $15.00  22.50 

Total   $1,379.00 

Approximate   cost  per   acre 140.00 

The  costs  indicated  above  are  considerably  higher  than  those  under 
which  many  fruit  growers  are  now  operating.  This  is  in  large  part 
due  to  recent  general  increases  in  cost  of  heating  equipment  and  to 
the  use  of  types  of  heaters  of  large  capacity.    There  are  many  oppor- 


62  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

tunities  for  reducing  the  initial  cost  and  fixed  charges  of  heating 
which  enterprising  and  enonomical  fruit  growers  can  employ  with 
advantage,  such  as  the  purchase  at  low  prices  of  used  equipment, 
the  repair  of  old  water  tanks  or  cisterns  for  storage  of  fuel,  the  use 
of  the  farm  wagon  or  truck  for  hauling  and  distributing  oil. 

Annual  Fixed  Charges  or  Overhead  Costs.  These  charges  include 
all  items  of  expense  that  are  properly  chargeable  against  orchard 
heating  regardless  of  whether  it  becomes  necessary  to  light  the  heaters 
or  not.  As  already  mentioned  the  most  important  of  these  are 
depreciation  and  interest  on  the  capital  invested  in  orchard  heating 
equipment.  Other  items  properly  classed  under  this  head  include 
the  cost  of  setting  the  heaters  in  place  and  filling  them  in  the  fall, 
of  emptying,  cleaning  and  painting  them  in  the  spring,  and  of  storing 
them  during  the  summer  months.  In  determining  annual  overhead 
costs  it  is  safe  to  allow  a  ten  per  cent  depreciation  rate  on  all  equip- 
ment and  an  annual  interest  charge  of  three  per  cent  on  the  initial 
cost  of  the  equipment  which  will  give  an  average  return  of  six  per 
cent  on  the  depreciated  value  over  a  ten  year  period.  Interest  on 
reserve  supplies  of  fuel  which,  if  properly  handled,  should  not 
depreciate  is  figured  at  six  per  cent. 

An  example  of  the  method  of  calculating  annual  overhead  costs 
is  given  in  table  17,  using  the  costs  for  an  orange  orchard  with  smoke- 
less heaters  as  stated  in  table  12.  The  same  method  may  be  used  to 
calculate  costs  for  any  other  case. 

TABLE  17 

Annual  Overhead  Cost  per  Acre 

Depreciation,  10  per  cent  on  $234.50 $23.45 

Interest,  3  per  cent  on  $234.50 7.04 

Interest  on  oil,  6  per  cent  on  $54.00 3.24 

Setting  heaters  in  field  and  filling 3.00 

Emptying  and  taking  up  in  spring 4.00 

Bepairs,    painting,    etc 2.00 

Total   $42.73 

The  last  three  of  these  items  are  typical  costs  taken  from  data 
secured  in  a  field  survey.  Obviously,  the  costs  of  these  items  vary 
widely  according  to  the  methods  used.  The  total,  while  rather  high 
for  oranges,  represents  typical  overhead  costs  where  expensive  equip- 
ment is  used.  Calculated  in  a  similar  manner  the  annual  overhead 
costs  per  acre  for  deciduous  fruits  where  lard  pail  heaters  are  used 
will  amount  to  about  $14.00. 


BULL.  398]  ORCHARD   HEATING    IN    CALIFORNIA  ()lj 

Operating  Costs.  Costs  of  operation  include  all  items  of  expense 
involved  in  the  actual  lighting  and  burning  of  the  heaters  and  should 
occur  only  in  seasons  when  damaging  low  temperatures  occur.  They 
may  vary,  therefore,  from  nothing  in  years  when  the  heaters  are  not 
lighted  to  relatively  high  figure  in  cold  seasons.  The  principal  items 
involved  are  the  costs  of  fuel,  of  lighting  and  regulating  the  heaters, 
of  refilling  them,  and  of  cleaning  and  dipping  or  painting  the  bowls 
and  stacks.  From  these  items  the  cost  of  operation  per  hour  of 
burning  can  be  calculated.  Cost  data  covering  these  various  opera- 
tions as  gained  from  a  recent  field  survey  showed  wide  variations. 
Average  oil  consumption  per  acre  per  hour  for  oranges  during  the 
winter  of  1921-25  varied  from  ten  to  thirty  gallons,  giving  a  variation 
in  cost  of  from  $.40  to  $1.20.  Filling  costs  varied  from  four-tenths 
of  a  cent  to  one  and  five-tenths  cents  per  gallon  of  oil,  and  labor  costs 
from  less  than  ten  cents  to  more  than  thirty  cents  per  acre  per  hour. 
Reasonable  operating  costs  for  oranges  are  apparently  about  $1.20 
per  acre  per  hour  as  an  average  for  the  season.  This  figure  is  based 
on  an  average  fuel  consumption  of  twenty  gallons  of  oil  per  acre  per 
hour  and  includes  costs  of  refilling.  The  cost  for  deciduous  fruits, 
assuming  an  oil  consumption  of  fifteen  gallons  per  acre  per  hour,  will 
be  about  $1.15  in  the  northern  part  of  the  state  where  oil  costs  more 
than  in  southern  California. 

Total  Costs.  The  total  yearly  cost  of  orchard  heating  is  made  up 
of  the  annual  overhead  charge  plus  the  cost  of  operations  for  the 
season.  From  the  data  presented  above  it  may  readily  be  seen  that 
marked  variation  in  orchard  heating  costs  may  be  expected  even  in 
a  given  locality.  By  exercising  good  judgment  and  foresight  there 
is  ample  opportunity  for  the  fruit  growers  to  materially  reduce  their 
costs  of  heating.  Community  oil  storage  close  at  hand  may  eliminate 
the  necessity  of  providing  storage  facilities  in  the  orchard.  If  the 
heaters  are  stored  full  of  oil  under  the  trees  the  costs  of  handling 
and  of  filling  and  emptying  may  be  materially  reduced.  The  purchase 
at  low  prices  of  used  equipment  may  markedly  lower  the  overhead 
cost. 


64  UNIVERSITY    OF    CALIFORNIA- — EXPERIMENT    STATION 


COMMON    MISTAKES    MADE    BY    GROWERS    IN    CONDUCTING 
HEATING    OPERATIONS 

Orchard  heating  is  an  operation  the  success  of  which  depends 
largely  upon  the  personal  efficiency  of  the  grower  who  undertakes  it. 
Whenever  the  personal  equation  enters  so  largely  into  any  operation 
mistakes  and  failures  are  inevitable.  Some  of  the  mistakes  which 
have  come  under  the  observation  of  the  authors  would  be  amusing 
were  it  not  for  the  disastrous  consequences  resulting  from  them. 
Some  of  the  more  common  causes  of  failure  will  be  discussed  in  this 
section. 

Insufficient  Equipment.  There  is  a  tendency  on  the  part  of  many 
growers  to  equip  on  the  basis  of  average  rather  than  of  extreme 
conditions.  During  a  survey  made  recently,  a  frequent  statement 
made  by  growers  was  that  fruit  had  been  lost  through  failure  to 
provide  a  sufficient  number  of  fires.  A  very  common  mistake  is  the 
failure  to  provide  extra  protection  around  the  borders  of  the  orchard. 

Inadequate  Knowledge  of  Temperatures.  Fuel  may  be  wasted 
or  fruit  lost  through  inadequate  knowledge  of  orchard  temperatures. 
Every  year  some  growers  are  found  who  have  no  thermometers  at  all, 
who  start  firing  when  neighbors  do  or  when  they  are  called  out 
by  some  protective  organization.  Others  place  dependence  on  inaccu- 
rate thermometers.  It  is  believed  that  the  inaccuracy  of  thermometers 
used  a  few  years  ago  was  largely  responsible  for  the  frequent  failures 
which  attended  heating  operations  at  that  time.  Many  growers  have 
the  thermometers  hidden  in  trees  where  it  is  difficult  if  not  impossible 
to  find  them  on  a  dark  night.  Sometimes  fruit  is  lost  after  sunrise 
because  the  heaters  are  extinguished  too  soon.  Heating  should  con- 
tinue until  a  check  thermometer  outside  of  the  heated  area  shows  a 
definite  rise  in  temperature. 

Torches  Improperly  Prepared.  Crops  have  been  lost  because  of 
inability  to  light  heaters  with  torches  stored  from  the  previous  season 
and  not  refilled.  The  evaporation  of  the  lighter  fractions  of  the  torch 
fuel  left  them  partially  filled  with  fuel  unsatisfactory  for  firing. 
Torches  should  be  freshly  filled  each  season  with  the  proper  mixture 
and  then  tested  before  they  are  needed. 

Heaters  Improperly  Assembled.  Heaters  will  not  burn  properly 
unless  the  bowl  covers  are  on  tight.  Growers  often  report  that  heaters 
go  out  after  lighting.  A  common  cause  of  this  trouble  is  improper 
placing  of  the  down  draft  tube  or  plate  in  heaters  which  have  this 


BULL.  398]  ORCHARD   HEATING   IN    CALIFORNIA  65 

attachment.  Several  growers  were  found  trying  to  operate  the 
"Supply  Company"  type  of  heater  with  the  draft  plate  missing. 
Tubes  or  plates  should  be  placed  so  as  to  carry  the  pilot  flame  toward 
the  center  of  the  heater  and  keep  it  concentrated  on  the  surface  of 
the  oil  as  the  oil  level  drops  in  the  bowl. 

Other  Mistakes.  Failure  to  remove  soot  from  the  stacks  or  to 
refill  the  heaters  after  a  few  hours'  burning  may  make  lighting  very 
difficult.  On  the  other  hand,  if  the  heaters  are  cleaned  too  thoroughly, 
especially  in  the  draft  parts,  or  are  filled  too  full  of  oil  they  will  be 
difficult  to  light  and  many  of  them  will  go  out  unless  given  special 
attention  in  lighting.  Regulating  tall  and  medium  stack  heaters  too 
soon  after  lighting  will  cause  many  of  them  to  go  out  or  to  burn 
unsatisfactorily.  The  failure  to  keep  the  stack  covers  on  tight  and 
all  openings  closed  permits  rain  water  to  get  into  the  oil.  This  causes 
frothing  and  sometimes  explosion  of  the  heaters.  Many  growers  have 
been  greatly  delayed  in  their  lighting  operations  the  first  dangerous 
night  of  the  season  because  the  draft  covers  and  regulating  hole  covers 
had  been  stuck  shut  with  fresh  paint.  On  wet  nights  these  heater 
parts  are  sometimes  frozen  and  can  be  opened  only  with  special  tools 
or  by  thawing  them  out  with  burning  torch  fuel.  Attempts  to  kick 
the  drafts  open  generally  result  in  bent  parts  which  make  regulating 
difficult. 

The  delegation  of  responsibility  for  heating  operations  to  incom- 
petent foremen  has  been  a  common  cause  for  failure.  This  highly 
important  and  very  expensive  orchard  operation  requires  intelligent 
supervision  and  careful  organization  before  the  danger  season.  It 
should  not  be  undertaken  by  persons  unwilling  to  undergo  the  incon- 
venience and  hard  work  necessary  to  insure  success. 


COMMUNITY    ORGANIZATION    FOR    ORCHARD    PROTECTION 

Some  of  the  aspects  of  community  interest  in  orchard  heating 
have  been  referred  to  in  a  previous  section.  In  certain  communities 
as  a  result  of  the  awakening  of  the  business  interests  to  the  recognition 
of  the  relation  of  orchard  heating  to  the  prosperity  of  the  district, 
arrangements  have  been  made  to  assist,  growers  in  financing  the  pur- 
chase of  heating  equipment  and  groups  of  growers  have  been  given 
aid  in  providing  facilities  for  storing  oil.  A  large  capacity  com- 
munity storage  tank  is  shown  in  figure  14. 

There  are  other  types  of  community  cooperation,  however,  which 
have  done  much  to  promote  orchard  heating  and  to  assist  citrus  fruit 


66 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


growers  in  protecting  their  crops.  Among  these  the  oldest  and  per- 
haps the  most  helpful  are  the  cooperative  frost  warning  organizations, 
of  which  the  Pomona  Valley  Frost  Protective  Association  is  one  of 
the  most  widely  known.  Organizations  of  this  type  of  which  there 
are  a  considerable  number,  have  greatly  reduced  the  risks  of  orchard 
heating  and  the  needless  consumption  of  oil  by  maintaining  a  reliable 
frost  patrol  system,  which  keeps  in  close  touch  with  temperature  con- 
ditions throughout  the  district  and  warns  the  individual  grower  by 
telephone  shortly  before  the  time  when  lighting  of  the  heaters  should 


T 


Fig.   ]4. — Large   community   oil   storage   tank. 


commence.  They  generally  work  in  close  cooperation  with  the  Fruit 
Frost  Service  of  the  Weather  Bureau  and  thereby  have  the  assurance 
that  their  service  is  based  on  authoritative  weather  forecasts.  The 
work  of  these  organizations  is  now  supplemented  by  radio  broadcast- 
ing from  Los  Angeles  of  the  Weather  Bureau  minimum  temperature 
predictions  for  the  different  citrus  producing  communities.  These 
predictions  are  for  the  "key  stations,"  which  are  selected  as  being 
typical  cold  spots  in  the  various  communities. 

The  organization  referred  to  has  been  particularly  active  in  pro- 
moting and  assisting  in  investigational  work  concerning  orchard 
heating  and  has  been  directly  responsible  for  much  of  the  progress 
made  in  the  heating  methods  and  equipment  of  Citrus  fruit  orchards. 
In  this  connection  it  should  be  mentioned  that  among  the  first  studies 


BULL.  398]  ORCHARD    HEATING    IX    CALIFORNIA  67 

ami  tests  of  orchard  heating  made  in  California  were  those  under- 
taken by  the  Riverside  Horticultural  Society  more  than  thirty  years 
ago. 

Frost  warning  organizations  are  financed  in  various  ways,  one  of 
the  commonest  being  pro-rating  the  expense  to  the  different  packing 
associations  in  the  district  served,  in  proportion  to  the  number  of 
cars  shipped  by  each.  There  is  still  a  considerable  field  for  expansion 
in  cooperative  frost  protection  work  of  this  type. 

Another  type  of  cooperative  organization  concerned  with  problems 
relating  to  orchard  heating  is  that  of  the  oil  purchasing  and  storage 
associations.  Some  of  these  represent  cooperation  within  a  given 
packing  association  or  between  different  packing  associations  within 
a  district  exchange,  while  others  are  entirely  separate  from  the 
marketing  organizations  and  are  organized  on  the  stock  issuance  basis. 
In  the  case  of  the  former,  the  local  packing  association  or  the  district 
exchange  owns  the  storage  and  distribution  facilities  and  the  oil  is 
purchased  through  the  subsidiary  purchasing  corporation  of  the 
central  marketing  agency.  All  costs  are  pro-rated  to  the  grower  on 
the  basis  of  a  charge  for  oil  determined  at  the  close  of  the  season's 
operations.  In  the  latter  type  of  organization  all  facilities  are  owTned 
by  the  corporation,  its  purchases  of  oil  made  in  the  the  open  market, 
and  the  costs  pro-rated  to  the  members  on  the  basis  of  the  amount  of 
oil  consumed  by  each.  In  order  to  minimize  disputes  over  the  return 
of  oil  by  members  of  oil  storage  organizations  it  is  believed  that 
weighing  the  oil  on  delivery  and  again  on  the  return  of  what  is  left 
at  the  end  of  the  season  with  a  sj^stem  of  dockage  for  dirty  oil  is 
to  be  preferred  over  other  systems  which  have  been  used. 

It  appears  not  at  all  unlikely  that  cooperation  in  orchard  heating 
will  receive  further  extension  among  Citrus  fruit  growers  in  such 
matters  as  the  delivery  of  oil  to  the  growers,  the  filling  and  refilling 
of  heaters,  and  perhaps  even  in  their  operation. 

Organizations  of  the  types  above  noted  have  done  much  to  place 
orchard  heating  on  the  sound  basis  that  it  now  occupies  in  the  citrus 
industry  of  California.  For,  in  addition  to  the  functions  for  which 
they  were  formed,  they  have  fostered  and  assisted  in  much  needed 
investigational  work,  have  aided  in  securing  the  important  frost  fore- 
casting service  of  the  Weather  Bureau,  and  have  done  much  to 
acquaint  the  residents  of  the  towns  and  cities  in  the  Citrus  districts 
with  the  importance  of  orchard  heating  as  a  practice  which,  although 
dirty,  laborious,  and  objectionable  alike  to  both  growTer  and  city 
resident,  is  necessary  for  the  prosperity  of  each. 


68  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


SUMMARY 

Severe  frost  damage  affecting  most  of  the  commercially  grown 
fruits  in  California  and  causing  heavy  losses  directly  to  the  fruit 
growers  and  indirectly  to  community,  state  and  nation  has  attracted 
the  attention  of  fruit  growers  as  an  orchard  management  problem  of 
great  importance.  In  fact,  frost  protection  is  now  considered  one 
of  the  essentials  of  success  in  the  growing  of  subtropical  fruits.  The 
present  investment  in  orchard  heating  equipment  in  California  is  not 
less  than  $2,500,000.00. 

It  is  unlikely  that  a  large  percentage  of  the  total  fruit  acreage  of 
California  will  ever  require  protection  against  frost  damage  but  where 
the  frost  hazard  is  great  orchard  heating  will  save  fruit  and  may  be 
a  very  profitable  operation,  and  change  failure  into  success.  It  is 
clear,  however,  that  it  is  justified  only  when  the  average  value  of 
the  crop  saved  is  in  excess  of  the  total  frost  protection  costs  and  this 
is  generally  the  case  only  where  production  per  acre  and  average  price 
are  high. 

Dangerously  low  temperatures  may  be  expected  in  the  winter  and 
spring  on  clear,  calm  nights  following  dry  or  cool  days.  Under  these 
conditions  a  relatively  thin  layer  of  air  near  the  ground  is  chilled 
by  radiation  and  conduction  until  it  is  below  the  freezing  point  of 
fruits  and  blossoms.  Above  this  thin  layer  the  air  is  warmer,  a 
phenomenon  known  as  temperature  inversion.  Temperature  inversion 
is  what  makes  heating  practicable  on  the  Pacific  Coast  because,  under 
these  special  atmospheric  conditions,  it  is  possible  to  raise  the  tem- 
perature of  a  thin  layer  of  air  just  a  few  degrees  and  not  expend 
heat  in  a  futile  attempt  to  "warm  up  all  out-of-doors." 

The  approach  of  conditions  causing  dangerously  low  temperatures 
can  be  determined  in  advance  and  the  United  States  Weather  Bureau 
is  able  to  issue  very  accurate  frost  forecasts.  In  many  fruit  growing 
sections  special  meteorological  observers  are  stationed  during  the 
danger  season.  These  men  belong  to  the  Fruit  Frost  Service,  which 
is  cooperative  between  the  Weather  Bureau  and  growers'  organiza- 
tions. They  issue  local  evening  forecasts  and  give  other  assistance 
to  growers  who  engage  in  heating. 

Frost  damage  can  be  prevented  both  by  the  prevention  of  heat 
losses  from  the  ground  and  by  the  addition  of  heat  to  the  air.  The 
latter  method  is  the  only  practicable  one  for  orchard  or  field  use.  It 
is  generally  necessary  to  raise  the  temperature  of  the  air  in  the 
orchard  only  a  few  degrees.     The  only  means  thus  far  developed  for 


BULL.  398]  ORCHARD   HEATING   IN    CALIFORNIA  69 

doing  this  is  the  addition  of  heat  from  a  relatively  large  number  of 
small  fires  per  acre.  This  may  be  accomplished  by  the  use  of  various 
kinds  of  orchard  heaters  burning  oil  or  other  fuels.  Success  is 
attained  only  by  keeping  a  sufficient  number  of  fires  burning  per  acre 
(sometimes  fifty  or  more)  during  the  entire  duration  of  temperatures 
below  the  danger  point.  Sufficient  heaters  and  fuel  must  be  supple- 
mented by  accessory  equipment  for  determining  temperatures  and 
handling  the  heating  operations. 

Equipment  for  orchard  heating  is  expensive  and  is  also  subject 
to  rapid  depreciation.    It  should  receive  careful  attention  at  all  times. 

There  are  a  great  many  factors  which  influence  the  resistance 
of  fruits  and  blossoms  to  frost  damage.  Information  is  gradually 
being  collected  on  these  factors  and  on  the  temperatures  which  will 
be  withstood  by  tender  plant  parts  under  different  conditions.  At  the 
present  time  long  experience  offers  the  best  guide  as  to  the  time  for 
lighting  heaters. 

Costs  of  orchard  heating  vary  from  less  than  fifteen  dollars  per 
acre  per  year  for  deciduous  fruits  in  locations  of  slight  frost  hazard 
to  over  one  hundred  dollars  for  citrus  orchards  in  cold  locations. 
Depreciation  and  interest  charges  account  for  a  large  proportion  of 
the  total  annual  cost  and  these  charges  go  on  from  year  to  year 
whether  the  heaters  are  lighted  or  not.  They  vary  according  to  the 
crop  to  be  protected  and  the  type  of  equipment  chosen.  Operating 
costs  vary  according  to  the  crop  to  be  protected  and  the  frost  hazard. 

The  personal  factor  is  of  the  greatest  importance  in  frost  pro- 
tection work  and  mistakes  frequently  occur  which  prevent  the  success 
of  heating  operations.  A  careful  study  of  the  problems  of  orchard 
heating  and  the  conditions  under  which  it  is  successful  as  well  as 
personal  attention  to  the  details  of  operation  are  essential  to  success. 

Orchard  heating  is  to  a  certain  extent  a  community  problem  in 
which  great  aid  is  rendered  by  community  organizations  which  pur- 
chase and  store  fuel,  call  growers  when  the  danger  point  is  reached, 
cooperate  with  the  Fruit  Frost  Service  of  the  Weather  Bureau,  and 
perform  other  helpful  functions. 


